Add a gallery example of setting line colors with a custom CPT (#774)

Co-authored-by: Dongdong Tian <[email protected]>

Author: willschlitzer

examples/gallery/line/line-custom-cpt.py

@@ -0,0 +1,32 @@
+"""
+Line colors with a custom CPT
+-----------------------------
+
+The color of the lines made by :meth:`pygmt.Figure.plot` can be set according to a
+custom CPT and assigned with the ``pen`` argument.
+
+The custom CPT can be used by setting the plot command's ``cmap`` argument to ``True``. The
+``zvalue`` argument sets the z-value (color) to be used from the custom CPT, and the line
+color is set as the z-value by using **+z** when setting the ``pen`` color.
+
+"""
+
+import numpy as np
+import pygmt
+
+# Create a list of values between 20 and 30 with at 0.2 intervals
+x = np.arange(start=20, stop=30, step=0.2)
+
+fig = pygmt.Figure()
+fig.basemap(frame=["WSne", "af"], region=[20, 30, -10, 10])
+
+# Create a custom CPT with the batlow CPT and 10 discrete z-values (colors)
+pygmt.makecpt(cmap="batlow", series=[0, 10, 1])
+
+# Plot 10 lines and set a different z-value for each line
+for zvalue in range(0, 10):
+    y = zvalue * np.sin(x)
+    fig.plot(x=x, y=y, cmap=True, zvalue=zvalue, pen="thick,+z,-")
+# Color bar to show the custom CPT and the associated z-values
+fig.colorbar()
+fig.show()