Replace deprecated IndexFormatter

docs/iris/src/whatsnew/latest.rst

@@ -97,6 +97,10 @@ This document explains the changes made to Iris for this release
   which was previously failing for some coordinate systems. See :issue:`3629`.
   (:pull:`3804`)
 
+* `@stephenworsley`_ changed the way tick labels are assigned from string coords.
+  Previously, the first tick label would occasionally be duplicated. This also
+  removes the use of Matplotlib's deprecated ``IndexFormatter``. (:pull:`3857`)
+
 
 💣 Incompatible Changes
 =======================

lib/iris/plot.py

@@ -249,7 +249,17 @@ def _string_coord_axis_tick_labels(string_axes, axes=None):
 
     ax = axes if axes else plt.gca()
     for axis, ticks in string_axes.items():
-        formatter = mpl_ticker.IndexFormatter(ticks)
+        # Define a tick formatter. This will assign a label to all ticks
+        # located precisely on  an integer in range(len(ticks)) and assign
+        # an empty string to any other ticks.
+        def ticker_func(tick_location, _):
+            tick_locations = range(len(ticks))
+            labels = ticks
+            label_dict = dict(zip(tick_locations, labels))
+            label = label_dict.get(tick_location, "")
+            return label
+
+        formatter = mpl_ticker.FuncFormatter(ticker_func)
         locator = mpl_ticker.MaxNLocator(integer=True)
         this_axis = getattr(ax, axis)
         this_axis.set_major_formatter(formatter)

lib/iris/tests/unit/plot/__init__.py

@@ -44,7 +44,7 @@ def tick_loc_and_label(self, axis_name, axes=None):
     def assertBoundsTickLabels(self, axis, axes=None):
         actual = self.tick_loc_and_label(axis, axes)
         expected = [
-            (-1.0, "a"),
+            (-1.0, ""),
             (0.0, "a"),
             (1.0, "b"),
             (2.0, "c"),