Make FuncFormatter work with non-fixed locator

The previous fix to make FuncFormatter work, in
https://github.com/mpld3/mplexporter/pull/67/files
and
mpld3/mpld3@e7fa282
had two issues still: 1) the mpl FuncFormatter API also takes the index as
second argument, which was missing. 2) it exported `tickvalues` only in
the FixedLocator case, so these were missing for non-fixed FuncFormatter
and hence the FuncFormatter codepath, which also requires them to be
present, was never hit with non-fixed locators.

Also add a test, and let tests import `matplotlib` via the
backend-setting mechanism, not only `plt`.

This fixes both. Again, this was figured out and helped with gpt-codex
and my careful review+cleanup. Codex even identified the two original
commits I'm linking above :)

Author: lucasb-eyer

mplexporter/tests/__init__.py

@@ -1,9 +1,6 @@
 import os
 
-MPLBE = os.environ.get('MPLBE', 'Agg')
-
-if MPLBE:
-    import matplotlib
+import matplotlib
+if MPLBE := os.environ.get('MPLBE', 'Agg'):
     matplotlib.use(MPLBE)
-
 import matplotlib.pyplot as plt

mplexporter/tests/test_utils.py

@@ -1,5 +1,5 @@
-from numpy.testing import assert_allclose, assert_equal
-from . import plt
+from numpy.testing import assert_, assert_allclose, assert_equal
+from . import plt, matplotlib
 from .. import utils
 
 
@@ -33,3 +33,23 @@ def test_axis_w_fixed_formatter():
     # NOTE: Issue #471
     # assert_equal(props['tickformat'], labels)
 
+
+def test_funcformatter_exports_major_ticklabels():
+    # Test both log and normal cases:
+    for scale, x, y in [
+        ("linear", [0, 1], [0, 1]),
+        ("log", [1, 10, 100], [0, 1, 2]),
+    ]:
+        fig, ax = plt.subplots()
+        ax.set_xscale(scale)
+        ax.plot([1, 10, 100], [0, 1, 2])
+        ax.xaxis.set_major_formatter(matplotlib.ticker.FuncFormatter(lambda x, pos: f"t{pos}"))
+
+        props = utils.get_axis_properties(ax.xaxis)
+        plt.close(fig)
+
+        assert_equal(props["scale"], scale)
+        assert_equal(props["tickformat_formatter"], "func")
+        assert_(props["tickvalues"] is not None)
+        assert_equal(len(props["tickvalues"]), len(props["tickformat"]))
+        assert_equal(props["tickformat"][:3], ["t0", "t1", "t2"])

mplexporter/utils.py

@@ -203,7 +203,7 @@ def _tick_format_props(formatter, tickvalues, labels):
     if isinstance(formatter, ticker.FixedFormatter):
         return list(formatter.seq), "fixed"
     if isinstance(formatter, ticker.FuncFormatter) and tickvalues:
-        return [formatter(value) for value in tickvalues], "func"
+        return [formatter(value, i) for i, value in enumerate(tickvalues)], "func"
     if not any(label.get_visible() for label in labels):
         return "", ""
     return None, ""
@@ -230,7 +230,7 @@ def get_axis_properties(axis):
     # Use tick values if appropriate
     locator = axis.get_major_locator()
     props['nticks'] = len(locator())
-    if isinstance(locator, ticker.FixedLocator):
+    if isinstance(locator, ticker.FixedLocator) or isinstance(axis.get_major_formatter(), ticker.FuncFormatter):
         props['tickvalues'] = list(locator())
     else:
         props['tickvalues'] = None