Add color gradients to polygons in psxy

[joa-quim]

Again with Claude assistance I was able to create another of the old wishes for coloring polygons. With the new -G+g option we can now created color gradients in polygons. So far only in psxy (it was a hell of a work for this one alone). Added a gradients.sh test but I'm unable to run it since something completely broke by ability to run bash shells on Windows.

An example

# Put this in the poly.txt file
1 1 red
5 1 green
5 5 yellow
2.5 5.5 orange
1 5 blue

gmt plot poly.txt -R0/6/0/6 -JX10c -Ba -G+g -W0.5p,black -png lixo

[Esteban82]

Great improvement. It works fine for me. Just two comments:

• I think that later we could add other ways to define colors (HSV, graygradients)
• At least in my PC, the headings in the test (gradients.sh) are too big.

[joa-quim]

It should accept all forms of specifying colors that we already have.

The test. The problem is that the images are too small. (5 cm each)

[joa-quim]

Now works in psxyz too. Added new gradients3d.sh test.

[Esteban82]

It should accept all forms of specifying colors that we already have.

Yes, that's true. But the rst files only mention using RGB, colornames, and CPT. I tried the H-S-V and C/M/Y/K formats and they worked.

However, for the grayscale format, an error occurs. It does not interpret them as grayscale but as values to use with a CPT.

cat << EOF > triangle.txt
0 0 0
3 0 100
1.5 2.6 200
EOF
   
gmt plot triangle.txt -R-1/4/-1/3 -JX10c -G+g -W0.5p,black -Baf -png gradient_gray

plot [ERROR]: Gradient option -G+g detected CPT format (x y z) but no color palette provided. Use -C<cpt> to specify a color palette.
psconvert [ERROR]: Unable to decode BoundingBox file /home/esteban82/.gmt/sessions/gmt_session.77159/psconvert_77209c.bb (maybe no non-white features were plotted?)

[joa-quim]

Yes, grays with a single value are not possible for the reason you found. This was a particular pain point when implementing the schema that guesses the file format and thus allows to do all with a single option (the +g) and not having to provide modifiers for each format.

But the rst files only mention using RGB, colornames

It mentions

2. Color name format: x y colorname where colorname can be a named color (e.g., "red", "blue"),
   hex color (e.g., "#FF0000"), or slash-separated RGB (e.g., "255/0/0").