Replace deprecated IndexFormatter (#3857)

* Replace deprecated IndexFormatter

* Blacken

* Add Whats New

* Clarify tick formatter comments

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"),