Make FuncFormatter work without FixedLocator.

mplexporter/tests/test_basic.py

@@ -16,9 +16,18 @@ def fake_renderer_output(fig, Renderer):
     return renderer.output
 
 
+# Separate function such that pytest fail reports are more readable,
+# because they print the values of the args, i.e. lines1, lines2.
+def _assert_output_equal_clean(lines1, lines2):
+    for l1, l2 in zip(lines1, lines2):
+        assert l1 == l2
+
+
 def _assert_output_equal(text1, text2):
-    for line1, line2 in zip(text1.strip().split(), text2.strip().split()):
-        assert line1 == line2
+    _assert_output_equal_clean(
+        [l.strip() for l in text1.strip().split('\n')],
+        [l.strip() for l in text2.strip().split('\n')],
+    )
 
 
 def test_lines():

mplexporter/tests/test_utils.py

@@ -1,4 +1,5 @@
 from numpy.testing import assert_allclose, assert_equal
+from matplotlib import ticker
 from . import plt
 from .. import utils
 
@@ -33,3 +34,23 @@ def test_axis_w_fixed_formatter():
     # NOTE: Issue #471
     # assert_equal(props['tickformat'], labels)
 
+
+def test_axis_w_funcformatter_autolocator():
+    fig, ax = plt.subplots()
+    ax.plot([0, 1], [0, 1])
+    ax.set_xlim(0, 1)
+
+    def formatter(value, position):
+        return f"{position}:{value:.1f}"
+
+    ax.xaxis.set_major_formatter(ticker.FuncFormatter(formatter))
+
+    props = utils.get_axis_properties(ax.xaxis)
+
+    assert_equal(props['tickformat_formatter'], "fixed")
+    assert props['tickvalues'] is not None
+    expected = [formatter(value, i)
+                for i, value in enumerate(props['tickvalues'])]
+    assert_equal(props['tickformat'], expected)
+
+    plt.close(fig)

mplexporter/utils.py

@@ -206,11 +206,9 @@ def get_axis_properties(axis):
 
     # Use tick values if appropriate
     locator = axis.get_major_locator()
-    props['nticks'] = len(locator())
-    if isinstance(locator, ticker.FixedLocator):
-        props['tickvalues'] = list(locator())
-    else:
-        props['tickvalues'] = None
+    tick_locs = list(locator())  # We'll use them later in some cases.
+    props['nticks'] = len(tick_locs)
+    props['tickvalues'] = tick_locs if isinstance(locator, ticker.FixedLocator) else None
 
     # Find tick formats
     props['tickformat_formatter'] = ""
@@ -235,9 +233,12 @@ def get_axis_properties(axis):
     elif isinstance(formatter, ticker.FixedFormatter):
         props['tickformat'] = list(formatter.seq)
         props['tickformat_formatter'] = "fixed"
-    elif isinstance(formatter, ticker.FuncFormatter) and props['tickvalues']:
-        props['tickformat'] = [formatter(value) for value in props['tickvalues']]
-        props['tickformat_formatter'] = "func"
+    elif isinstance(formatter, ticker.FuncFormatter):
+        # It's impossible for JS to re-run our function, so run it now and save as Fixed.
+        if props['tickvalues'] is None:
+            props['tickvalues'] = tick_locs
+        props['tickformat'] = [formatter(value, i) for i, value in enumerate(props['tickvalues'])]
+        props['tickformat_formatter'] = "fixed"
     elif not any(label.get_visible() for label in axis.get_ticklabels()):
         props['tickformat'] = ""
     else: