Add a gallery example for a double y-axes graph

examples/gallery/plot/double-y-axes.py

@@ -0,0 +1,70 @@
+"""
+Double Y-axes
+-------------
+
+The ``frame`` parameter of the plotting methods of the :class:`pygmt.Figure`
+class can control which axes should be plotted and possibly show annotations
+and tick marks. By default, all the 4 axes are plotted, along with annotations
+and tick marks (denoted **W**, **S**, **E**, **N**). Lower cases (**w**, **s**,
+**e**, **n**) can be used to denote to only plot the axes with tick marks. We
+can also only plot the axes without annotations and tick marks using **l**
+(left axis), **r** (right axis), **t** (top axis), **b** (bottom axis).
+"""
+
+import numpy as np
+import pygmt
+
+# Generate common x values and two kinds of y values
+x = np.linspace(1.0, 9.0, num=9)
+y1 = x
+y2 = x ** 2 + 110
+
+fig = pygmt.Figure()
+
+# Plot y1
+# The left (W) and bottom (S) axes are plotted with annotations and tick marks
+fig.basemap(
+    region=[0, 10, 0, 10],
+    projection="X5c/5c",
+    frame=["WS", "xaf+lx", "yaf+ly1"],
+)
+# Plot the line for y1
+fig.plot(
+    x=x,
+    y=y1,
+    pen="1p,blue",
+)
+# Plot points for y1
+fig.plot(
+    x=x,
+    y=y1,
+    style="c0.2c",
+    color="blue",
+    label="y1",
+)
+
+# Plot y2
+# The right axis (E) is plotted with annotations and tick marks
+# The top axis (t) is plotting without annotations and tick marks
+fig.basemap(
+    region=[0, 10, 100, 200],
+    frame=["Et", "yaf+ly2"],
+)
+# Plot the line for y2
+fig.plot(
+    x=x,
+    y=y2,
+    pen="1p,red",
+)
+# Plot points for y2
+fig.plot(
+    x=x,
+    y=y2,
+    style="s0.28c",
+    color="red",
+    label="y2",
+)
+
+fig.legend(position="JTL+jTL+o0.1c", box=True)
+
+fig.show()