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plot(x, y, G=value, C=cmap) gives strange behavior for individual points #417
Description
Description of the problem
In plot(), using G=value and C=cmap to interpret a color from a CMAP for a single value doesn't work in the same way that a list/array of values does. Interpreting a single value from a color map would be useful for plotting a bunch polygons with different colors (ex: for coulomb stress changes on fault segments).
- For plotting arrays of points, G returns an array of colors extracted from the cmap, as it should
- For plotting single polygons, G returns foreground or background color. wrong?
- For plotting single points, G returns 0. wrong?
In the single-element case, plot() requires G=str, which is also counter-intuitive.
Either way, the result makes it difficult to plot custom-colored polygons with a colormap
(usually I would just extract a color for each polygon and plot each one separately, calling plot() in a loop). Is there another way to plot polygons with cmap colors?
In the example code, I'm plotting some dummy data, once as an array of points, and once in a for-loop with points and polygons in individual plot() calls. The cmap behavior is quite different.
Full code that generated the error
import pygmt
# Make CPT and Fake Data
pygmt.makecpt(C="jet",T="-1.0/1.0/0.05",H="mycpt.cpt");
xdata = [-123, -122.5, -122.0, -121.5];
ydata = [36.5, 37, 37.5, 38];
zdata = [-0.7, -0.25, 0.25, 0.7];
# Batch Calling : works
fig = pygmt.Figure()
title="+t\"Batch to CMAP - Works for Points.\"";
fig.coast(region=[-124, -120, 35, 39], projection="M7i", B=[title,"1.0"],shorelines="1.0p,black");
fig.colorbar(D="jBr+w3.5i/0.2i+o5.5c/1.5c+h",C="mycpt.cpt",I="0.8",G="-1.0/1.0",B=["x"+str(0.2),"y+L\"KPa\""]);
fig.plot(x=xdata, y=ydata, color=zdata, style="c1.8c", pen="thick,black", C="mycpt.cpt");
fig.savefig("Working.png");
# Single Calls Only : doesn't do the right thing
fig = pygmt.Figure()
title="+t\"Single Calls to CMAP - Odd.\"";
fig.coast(region=[-124, -120, 35, 39],projection="M7i",shorelines="1.0p,black",B=[title,"1.0"]);
fig.colorbar(D="jBr+w3.5i/0.2i+o5.5c/1.5c+h",C="mycpt.cpt",I="0.8",G="-1.0/1.0",B=["x"+str(0.2),"y+L\"KPa\""]);
for i in range(len(xdata)): # just looping through the data
fig.plot(x=xdata[i], y=ydata[i], G=str(zdata[i]), style="c1.8c", pen="thick,black", C="mycpt.cpt"); # plotting a point with associated cmap data
xdata_long = [xdata[i]+0.2, xdata[i]+0.3, xdata[i]+0.3, xdata[i]+0.2, xdata[i]+0.2]; # building a polygon
ydata_long = [ydata[i], ydata[i], ydata[i]+0.1, ydata[i]+0.1, ydata[i]]; # building a polygon
fig.plot(x=xdata_long, y=ydata_long, G=str(zdata[i]), pen="thick,black", C="mycpt.cpt"); # plotting the polygon with associated cmap data
fig.savefig("Not_Working.png");System information
Mac OSX Mojave, miniconda3 package manager, python3.6. I installed pygmt in a brand new python environment today.
Activity
welcome commentedon Apr 16, 2020
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weiji14 commentedon Apr 17, 2020
Right, bear with me for a little bit while I get my head around this. I think this is similar to the problem we had in #376.
To colour your points in the for-loop, the
Gargument needs to be given alistinstead of astr(which is inconsistent with the documentation). I.e.G=[zdata[i]].Edit: To colour the polygon, you can set the
Zargument to something likeZ="f0.7"which would base the fill colour of the polygon on your provided cpt. I've updated the code below.produces:
I'll need to work out how to colour a filled polygon, let me go and have lunch first :)Still, I think we might need to extend the fix in #376 to includeG/coloras well, or change the documentation to make it clear, or do both.kmaterna commentedon Apr 17, 2020
Thank you so much for your quick reply- this is exactly it! 🎉🎉🎉
I guess this is like the -Zval in GMT, but I would have had trouble figuring this one out on my own. Thanks!!
weiji14 commentedon Apr 18, 2020
Yes it does get a bit confusing, but glad it helped. The
-Zsyntax seems to be new in GMT 6.0.0, and it might change in GMT 6.1.0 (see GenericMappingTools/gmt/issues/3131 for context) to make colouring the fill and outline more intuitive.I'll leave this issue open for now until we clear up the
plotdocumentation.weiji14 commentedon Feb 17, 2021
Documentation for
plothas been revised significantly in PyGMT v0.3.0 (see https://www.pygmt.org/v0.3.0/api/generated/pygmt.Figure.plot.html) which uses GMT 6.1.1, and long alias zvalue (Z) was added in #666. It might still be a bit confusing getting to know how color (-G) and zvalue (-Z) works, but I'll close this issue now and we can open up a fresh issue later (if there's demand) to add a gallery example for using -G/-Z in plot to colour individual points.