Hints to PyNGL/PyNIO exercises

Basic graphics exercise hints
Color exercise hints
Title exercise hints
XY plot exercise hints (set 1)
XY plot exercise hints (set 2)
Contour plot exercise hints
Contours over maps exercise hints (set 1)
Contours over maps exercise hints (set 2)
Vector plot exercise hints
Primitives exercise hints (set 1)
Primitives exercise hints (set 2)
Primitives exercise hints (set 3)
Paneling exercise hints

Basic graphics exercise hints

To have the script generate a PS or PDF file, change "x11" to "ps" or "pdf" in the call to Ngl.open_wks. Change the second argument to indicate the name of the PS or PDF file to write to.
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To rotate the image for PS or PDF output, you need to use the resource wkOrientation. Read the documentation on Ngl.open_wks for information on how to set this resource.
Setting a resource requires creating a resource list using call to "Ngl.Resources()".
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nglMaximize is True by default, which forces the plot to be maximized in the output device you're using. This means if you are drawing to a PS file that will be sent to an 8-1/2 x 11 piece of paper, the plot may be rotated to landscape mode if the plot is wider than it is high (which should be the case here). If you don't want the plot maximized in the frame, set this resource to False.
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Drawing another plot is simple. Just add another call to Ngl.y, and pass it the new set of y points.
Note that if your output is going to an X11 window, you will need to click on the X11 window with your left mouse button to advance the frame to the second picture, and then again to close the window.
If your output is going to a PS or PDF file, each plot will appear on a separate page.
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Setting nglFrame to False causes the frame not to be advanced, and hence if you draw a second plot, it will be drawn on the same frame (or page), as the previous plot.
To set nglFrame to False for one plot and not the other, set it to False before the first call, and then back to True for the second call. Or, you can just delete it before you generate the second plot.
If your output is going to an X11 window, you will only see one X11 window pop up, and the second plot will be drawn to the same window.
If your output is going to a PS or PDF file, both plots will be draw on the same page.
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Use the viewport vpXF, vpYF, vpWidthF, and vpHeightF, to reposition and resize individual plots. The values of the vpXF and vpYF resources represent the top left corner of the plot.
To advance the frame manually, call the Ngl.frame procedure. You must advance the frame after you are done drawing to it, because otherwise the output graphics file(s) will not be closed properly.
Note that when you use "vp" resources to position plot(s) on a page, the nglMaximize resource will no longer have effect.
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Note: if your plots are the exact same size, and you want to panel them on a page, then use Ngl.panel to do this.

Color exercise hints

Use the procedure Ngl.draw_colormap to draw the current color table. Each color will have its corresponding index included. Color index 0 represents the background color, and 1 represents the foreground color.
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Use the procedure Ngl.define_colormap to change the color table (also known as a "color map"). It is generally a good idea to have only one color table per frame (page).
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Use the function Ngl.new_color to add a new color to a color table. The existing color table must have less than 256 colors, as this is the maximum number of colors you can have. The color will be added to the end of your color table.
The color gray is represented by RGB triplets that have the same values, like (0.1,0.1,0.1) or (0.85,0.85,0.85). The larger the values, the lighter the shade of gray.
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Use the procedure Ngl.define_colormap to create a new color table using RGB triplets. Since you need to create an n x 3 array, use NumPy's "array" function to create it.
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There are other ways to create your own color table. For more information, see the short write up on creating your own color table.
Use the procedure Ngl.define_colormap to create a new color table using named colors.
To see all the color names and their corresponding colors, see the named colors gallery.
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Title exercise hints

Resources for changing attributes of the main title start with "tiMain" and are listed alphabetically in the title resource page.
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Resources for changing attributes of the X or Y axis titles start with "tiXAxis" and "tiYAxis". The resources for the main title, X axis title, and Y axis title have the same structure, i.e. "tiMainFont", "tiXAxisFont", and "tiYAxisFont". This makes it easier to remember resource names.
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If you change the orientation of text, this may also change the direction of the text. This means you need to set two resources in order to change the orientation of the Y axis title. You unfortunately can't have a title on both Y axes using the "ti" resources. You would need to use Ngl.add_text if you wanted to add an additional title to one of the axes.
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XY plot exercise hints (set 1)

Resources for changing attributes of an XY line start with "xyLine" and are listed alphabetically in the XY resource page.
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All line thickness resources have names like xyLineThicknessF, xyLineThicknesses, cnLineThicknessF, cnLineThicknesses, gsLineThicknessF, etc. The default is usually 1.0, so a value of 2.0 makes the line twice as thick. Sometimes you need to make lines thicker so they'll look better in a PS or PDF file.
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Changing an XY plot so that markers are used instead of lines involves changing a "mode" resource. To then change the look of a marker, there are a series of "xyMarker" resources you can set. You can see a list of predefined markers in the marker table.
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Try playing with these and other xyMarker resources to change the marker, color, size, etc.
If you don't like any of the predefined markers, you can define your own using Ngl.new_marker. This function allows you to use any of the characters in any of the font tables as a marker. In the script below, the character "p" is used from font table 35.
If you don't want to plot every value, you can use the "::" syntax to subscript your data array. Using "::2" will grab every other point, while, "::3" will grab every third point.
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Try playing with these and other xyMarker resources to change the marker, color, size, etc.

XY plot exercise hints (set 2)

Use Ngl.fspan to generate the X array. In order to use it in a call to Ngl.xy, it must be the same length as its corresponding Y array.
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You can reorder the dimensions by using the NumPy function "transpose". If you try to plot the array without transposing it, PyNGL will try to plot 500 curves each with 4 points, and you will get lots of error messages about too many legends being created.
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Some resources have both scalar and plural representations, like xyLineColor and xyLineColors. If you use a scalar resource for a multi-curve plot, the value will apply to all curves.
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Changing the area that the data is displayed in involves a transformation of the data to a new space, so you need to use Transformation resources to change the axes limits.
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When you create an XY plot, an automatic legend is created for you, although it is not drawn by default. To force the draw of the legend, set a PlotManager resource called pmLegendDisplayMode. The PlotManager is something that manages additional annotations of a plot, like labelbars, titles, and legends.
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Legends are somewhat confusing in PyNGL. To change various aspects of a legend, you may need to use xy, pm, and lg resources. For this particular exercise, no "lg" resources are involved.
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Contour plot exercise hints

There are three ways you can set contour levels: 1) let PyNGL do it automatically, 2) set a minimum level, maximum level, and a level spacing, or 3) set an array of contour levels.
The default behavior is #1. To do #2 or #3, you need to set the cnLevelSelectionMode resource to the "mode" you want, and then set the other resources depending on the mode you choose.
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Whenever you have a single and plural version of a resource, like "cnLineColor" and "cnLineColors", there's usually a corresponding "Mono" resource ("cnMonoLineColor") that takes a logical True/False value. This allows you to specify whether to use the singular or plural version of the resource to apply to that specific plot feature.
You need to know whether the corresponding Mono resource's default is True or False to know whether you need to change it.
Other contour line resources start with "cnLine".
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Use contour resources ending in "On" to toggle certain plot features on and off.
In PyNGL, the contour fill colors are selected such that they span the full color map (nglSpreadColors=True). [Note: if you only want a particular part of your color map spanned, you can set the resources nglSpreadColorStart and nglSpreadColorEnd to the desired start and end indexes.]
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Use the "ti" resources to add and change titles. You can use a function code to change the font.
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A labelbar is added automatically when contour fill is set. By default, it is vertical; use a labelbar resource to change the orientation to horizontal.
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Tickmark resources can be set for XY, vector, streamline, and contour plots, and hence they have their own set of resources that start with the letters "tm".
There are three ways you can change the labels on a tickmark axis: 1) let PyNGL do it automatically, 2) set a minimum and maximum tickmark value and a tickmark spacing, or 3) set arrays of tickmark values and labels you want at those values. For this exercise, you will use #2 for the Y axis, and #3 for the X axis.
Each of the four axes has its own set of tickmark resources. By default, however, the right/left axes will use the same resources values, as will the top/bottom axes (unless instructed otherwise). To set explicit labels for the bottom X axis, look for the "tmXB" resources, and for the left Y axis, look for "tmYL".
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Changing certain aspects of a labelbar involves using PlotManager resources. Moving objects closer or parallel to other objects sometimes involves the use of "Orthogonal" and "Parallel" resources.
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Contours over maps exercises (set 1)

To draw your contours over a map, call Ngl.contour_map instead of Ngl.contour. This function defaults to a cylindrical equidistant map projection. Again, the contours will not be in the correct location. This is because we haven't given the function any latitude/longitude information yet.
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If you add a "print(tf)" to your script, this will print information about what's on the netCDF file. You'll see that it contains a "lat" and "lon" array. Use these arrays as values to the two scalar field resources sfYArray and resources sfXArray
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Most of the resources used for this example were discussed in the contour plot exercises section above.
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Most of the resources used for this example were discussed in the contour plot exercises section above. You want to use a mix of PlotManager and LabelBar resources
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By default, lbLabelAutoStride is set to True (it's False for NCL). To override this and control the number of labels on the labelbar yourself, look for another LabelBar "Stride" resource.
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First, you will need to add "tan" to your color map using Ngl.new_color. You can look at the named color tables to find a tan color you like. Then, look up this named color in the rgb.txt file. Divide the three numbers by 255 to get the RGB triplet to use.
Make sure that this new tan color is not used in your contour colors, by setting the special nglSpreadColorEnd resource.
To control the color of land and ocean, you can use individual resources like mpLandFillColor and mpOceanFillColor, but it's easier to use the single resource mpFillColors. Set the land color part of this resource to -1 make it transparent.
To control the draw order of various aspects of a plot, check the resource list for that object for "DrawOrder" type resources. These can be set to "PreDraw", "Draw", or "PostDraw" in order to control which phase certain objects are drawn in.
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Contours over maps exercises (set 2)

Use scalar field resources to set the lat, lon arrays, and the missing value.
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There are several ways to zoom in on a map. See the documentation for mpLimitMode for more information. The most common limit mode is "LatLon".
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Most of the resources used for this example were discussed in the contour plot exercises section above. Note that in this case, we are not setting a minimum or maximum contour level; we are only setting the spacing. This is possible when you use a cnLevelSelectionMode setting of "ManualLevels".
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Use the mpProjection resource to change the map projection. The map projection may not look right, so you need to change the center latitude and longitude values.
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The map resources of interest here all start with "mpGrid". To turn off the top tickmarks, look at tmXTOn.
Turn off the top tickmarks.
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Vector plot exercise hints

Making the vectors multi-colored involves the use of one of the "vcMono" resources.
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Changing the reference magnitude is probably one of the most common things you'll want to do to a vector plot. Check out the vcRefMagnitudeF resource as a start.
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Use Ngl.define_colormap to change the color map. The nglSpreadColors resource is already True by default, so the full color map will be spanned. See the contour exercises above to see how to span only certain sections of a color map.
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Moving the reference annotation box involves the use of a vector orthogonal resource to move the box up and down, and a parallel resource to move the box left and right. Generally you have to use trial and error to get the box where you want it.
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The default "mode" for setting vector levels is "AutomaticLevels", where PyNGL tries to pick "nice" levels based on the magnitude of your data. In order to set the levels yourself, see the vcLevelSelectionMode resource.
Use tiMainString to change the title.
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Just as you use scalar field resources for a contour plot, you can use vector field resources for a vector plot.
Use PlotManager and LabelBar resources to change the size of the labelbar and the labels.
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There are 64 timesteps on the file. Instead of going to an "x11" window, then, it might be better to go to an NCGM file and use idt to animate the NCGM file.
Use the "all" function to test for all missing data.
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Primitives exercise hints (set 1)

You need to set nglFrame to False so that you can draw the marker before the frame is advanced. Use the Ngl.polymarker procedure to draw the marker using plot coordinates.
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To set some marker resources, use the "gs" (GraphicStyle) resources that start with "gsMarker".
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You can use the same resource list ("mkres") to change the markers for each set that you draw. To draw more than one marker of the same style, size, and color, pass an array of X and Y values to Ngl.polymarker.
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This exercise produces the exact same plot as the previous exercise, only using Ngl.add_polymarker instead of Ngl.polymarker. When you use any of the "Ngl.add_polyxxx" functions, you must use a unique variable on the lefthand side for each call.
You should also turn off the initial drawing of the plot and draw the plot manually after you add the markers.
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Primitives exercise hints (set 2)

Use the Ngl.add_polyline function to add the lines using lat/lon coordinates. You need to set nglFrame to False so that you can draw the map with the attached lines before the frame is advanced. To set some lines resources, use the "gs" (GraphicStyle) resources that start with "gsLine".
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Use the Ngl.add_polygon function to draw the polygon. You can use the same lat/lon coordinates that you used for the box lines. To set polygon resources, use the "gs" (GraphicStyle) resources that start with "gsFill".
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See the "DrawOrder" resources in the list of map resources for a way to change the drawing order of the outlines and grid lines so they get drawn before the polygon is drawn.
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As with the above exercise, you can use "gsFill" resources to change the fill pattern and color of your polygon. Click here to see a table of the available fill patterns.
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Use the Ngl.add_polymarker function to add the polymarkers. Note that when you use the "Ngl.add_xxx" functions, the variables on the lefthand side must be unique, and they must not be deleted.
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Use the Ngl.add_text function to add the text. The text resources start with "tx" (TextItem). Again, the variable on the lefthand side must be unique.
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Primitives exercise hints (set 3)

As mentioned, you want to use Ngl.xy inside a loop to draw each bar. This is a bit redundant, because the axes and tickmarks will get redrawn each time. Since we are only drawing eight bars, it's not too bad timewise.
You will need to set the special resource nglFrame to False so you can draw each bar before the frame is advanced. Add a call to Ngl.frame after the loop to make sure the frame is advanced properly.
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Use tm resources to turn off the tickmarks for a particular axis.
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Use Ngl.define_colormap to define a color map, and Ngl.polygon to draw a filled polygon.
Use a GraphicStyle resource to set the polygon fill color.
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Use a viewport resources to control the location and size of the plot. Since the special resource nglMaximize is set to True by default, it will recalculate the viewport resources. Set this to False so you can change the viewport resources yourself.

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Whenever you want to add polylines, polymarkers, polygons, or text ("primitives") outside of the plot area, you can call Ngl.draw_ndc_grid to draw vertical and horizontal lines at various NDC values. This will help you determine what NDC values to use for your primitives.
You want to call Ngl.draw_ndc_grid before you advance the frame. You can call it before or after you draw a plot, depending on whether you want the lines drawn under or on top of the plot. For this exercise, draw the grid before you draw the plot.
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Use the function Ngl.polyline_ndc to draw a box using NDC values. For changing attributes of the lines, use GraphicStyle resources that start with "gsLine".
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There's not much different with this script from the previous script, except that each box should be drawn inside a python "for" loop.
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Use the function Ngl.text_ndc to draw text strings in NDC space. Text resources start with "tx".
The default is to center each text string about the X,Y position, so to left-justify it, you need to set the txJust resource.
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The default line color is the foreground color (black in this case), so there's no need to set any resources for the Ngl.polyline_ndc call. It works better if you draw the polylines after the polygons, so the polylines are not obscured by the polygons.
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Use tmYMajorGrid resources to turn on and change horizontal grid lines for the major tickmarks.
Since there's no way to turn tickmarks off, you turn them off by setting their lengths equal to zero (see tmYLMajorLengthF for a start).
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Paneling exercise hints

To create an array of plots, create an empty list and use the "append" method to append each plot to the array as you create it.
To make sure each dataset is unique, you can use a different seed in the Ngl.generate_2d_array function.
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Use the Ngl.panel to panel the individual plots.
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The nglDraw and nglFrame resources control the drawing of the individual plots.
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See the resource cnLevelSelectionMode.
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This example also uses the cnLevelSelectionMode resource, but sets it to a different value.
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To turn off individual labelbars, see lbLabelBarOn. To turn on a common labelbar in a panel plot, see nglPanelLabelBar.
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See the resources nglPanelFigureStrings and nglPanelYWhiteSpacePercent. Note there is a similar resource for white space in the X direction.
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To turn off tickmarks, you need to set tickmark resources for the individual plots. See tmXTOn and tmYROn.
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To add a main title, use the Ngl.text_ndc function to draw a string in NDC space. Since there is no room for a title, you will need to make room by using the special nglPanelTop resource (for future reference, there are corresponding Bottom, Left, and Right resources as well).
You can use a function code to change the font. See the font tables to find a bold font.
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