Basic Usage tutorial: add additional axes section

Secondary and twin axes are rather important (although not necessarily
very basic) so they should be shortly mentioned with links to the API
docs and some more detailed examples.

Author: StefRe

tutorials/introductory/usage.py

@@ -367,7 +367,8 @@ def my_plotter(ax, data1, data2, param_dict):
 #
 # Each Axes has two (or three) `~.axis.Axis` objects representing the x- and
 # y-axis. These control the *scale* of the Axis, the tick *locators* and the
-# tick *formatters*.
+# tick *formatters*. Additional Axes can be attached to display further Axis
+# objects.
 #
 # Scales
 # ------
@@ -449,6 +450,34 @@ def my_plotter(ax, data1, data2, param_dict):
 # numbers or dates.  If you pass 1000 strings, Matplotlib will think you
 # meant 1000 categories and will add 1000 ticks to your plot!
 #
+#
+# Additional Axis objects
+# ------------------------
+#
+# Plotting data of different magnitude in one chart may require
+# an additional y-axis. Such an Axis can be created by using
+# `~.Axes.twinx` to add a new Axes with an invisible x-axis and a y-axis
+# positioned at the right (analogously for `~.Axes.twiny`). See
+# :doc:`/gallery/subplots_axes_and_figures/two_scales` for another example.
+#
+# Similarly, you can add a `~.Axes.secondary_xaxis` or
+# `~.Axes.secondary_yaxis` having a different scale than the main Axis to
+# represent the data in different scales or units. See
+# :doc:`/gallery/subplots_axes_and_figures/secondary_axis` for further
+# examples.
+
+fig, (ax1, ax3) = plt.subplots(1, 2, figsize=(8, 2.7), constrained_layout=True)
+l1, = ax1.plot(t, s)
+ax2 = ax1.twinx()
+l2, = ax2.plot(t, range(len(t)), 'C1')
+ax2.legend([l1, l2], ['Sine (left)', 'Straight (right)'])
+
+ax3.plot(t, s)
+ax3.set_xlabel('Angle [°]')
+ax4 = ax3.secondary_xaxis('top', functions=(np.rad2deg, np.deg2rad))
+ax4.set_xlabel('Angle [rad]')
+
+##############################################################################
 # Color mapped data
 # =================
 #