matplotlib
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+"""
+=========================
+Multilevel (nested) ticks
+=========================
+
+Sometimes we want another level of tick labels on an axis, perhaps to indicate
+a grouping of the ticks.
+
+Matplotlib does not provide an automated way to do this, but it is relatively
+straightforward to annotate below the main axis.
+
+These examples use `.Axes.secondary_xaxis`, which is one approach. It has the
+advantage that we can use Matplotlib Locators and Formatters on the axis that
+does the grouping if we want.
+
+This first example creates a secondary xaxis and manually adds the ticks and
+labels using `.Axes.set_xticks`.  Note that the tick labels have a newline
+(e.g. ``"\nOughts"``) at the beginning of them to put the second-level tick
+labels below the main tick labels.
+"""
+
+import matplotlib.pyplot as plt
+import numpy as np
+
+import matplotlib.dates as mdates
+
+rng = np.random.default_rng(19680801)
+
+fig, ax = plt.subplots(layout='constrained', figsize=(4, 4))
+
+ax.plot(np.arange(30))
+
+sec = ax.secondary_xaxis(location=0)
+sec.set_xticks([5, 15, 25], labels=['\nOughts', '\nTeens', '\nTwenties'])
+
+# %%
+# This second example adds a second level of annotation to a categorical axis.
+# Here we need to note that each animal (category) is assigned an integer, so
+# ``cats`` is at x=0, ``dogs`` at x=1 etc.  Then we place the ticks on the
+# second level on an x that is at the middle of the animal class we are trying
+# to delineate.
+#
+# This example also adds tick marks between the classes by adding a second
+# secondary xaxis, and placing long, wide ticks at the boundaries between the
+# animal classes.
+
+fig, ax = plt.subplots(layout='constrained', figsize=(7, 4))
+
+ax.plot(['cats', 'dogs', 'pigs', 'snakes', 'lizards', 'chickens',
+         'eagles', 'herons', 'buzzards'],
+        rng.normal(size=9), 'o')
+
+# label the classes:
+sec = ax.secondary_xaxis(location=0)
+sec.set_xticks([1, 3.5, 6.5], labels=['\n\nMammals', '\n\nReptiles', '\n\nBirds'])
+sec.tick_params('x'
8000
, length=0)
+
+# lines between the classes:
+sec2 = ax.secondary_xaxis(location=0)
+sec2.set_xticks([-0.5, 2.5, 4.5, 8.5], labels=[])
+sec2.tick_params('x', length=40, width=1.5)
+ax.set_xlim(-0.6, 8.6)
+
+# %%
+# Dates are another common place where we may want to have a second level of
+# tick labels.  In this last example, we take advantage of the ability to add
+# an automatic locator and formatter to the secondary xaxis, which means we do
+# not need to set the ticks manually.
+#
+# This example also differs from the above, in that we placed it at a location
+# below the main axes ``location=-0.075`` and then we hide the spine by setting
+# the line width to zero.  That means that our formatter no longer needs the
+# carriage returns of the previous two examples.
+
+fig, ax = plt.subplots(layout='constrained', figsize=(7, 4))
+
+time = np.arange(np.datetime64('2020-01-01'), np.datetime64('2020-03-31'),
+                 np.timedelta64(1, 'D'))
+
+ax.plot(time, rng.random(size=len(time)))
+
+# just format the days:
+ax.xaxis.set_major_formatter(mdates.DateFormatter('%d'))
+
+# label the months:
+sec = ax.secondary_xaxis(location=-0.075)
+sec.xaxis.set_major_locator(mdates.MonthLocator(bymonthday=1))
+
+# note the extra spaces in the label to align the month label inside the month.
+# Note that this could have been done by changing ``bymonthday`` above as well:
+sec.xaxis.set_major_formatter(mdates.DateFormatter('  %b'))
+sec.tick_params('x', length=0)
+sec.spines['bottom'].set_linewidth(0)
+
+# label the xaxis, but note for this to look good, it needs to be on the
+# secondary xaxis.
+sec.set_xlabel('Dates (2020)')
+
+plt.show()