Merge pull request #20180 from meeseeksmachine/auto-backport-of-pr-19…

…876-on-v3.4.x

Backport PR #19876 on branch v3.4.x (FIX: re-order unit conversion and mask array coercion)

lib/matplotlib/axes/_axes.py

@@ -5651,9 +5651,10 @@ def _pcolorargs(self, funcname, *args, shading='flat', **kwargs):
         if len(args) == 3:
             # Check x and y for bad data...
             C = np.asanyarray(args[2])
-            X, Y = [cbook.safe_masked_invalid(a) for a in args[:2]]
             # unit conversion allows e.g. datetime objects as axis values
+            X, Y = args[:2]
             X, Y = self._process_unit_info([("x", X), ("y", Y)], kwargs)
+            X, Y = [cbook.safe_masked_invalid(a) for a in [X, Y]]
 
             if funcname == 'pcolormesh':
                 if np.ma.is_masked(X) or np.ma.is_masked(Y):

lib/matplotlib/tests/test_axes.py

@@ -1637,6 +1637,36 @@ def test_boxplot_dates_pandas(pd):
     plt.boxplot(data, positions=years)
 
 
+def test_pcolor_regression(pd):
+    from pandas.plotting import (
+        register_matplotlib_converters,
+        deregister_matplotlib_converters,
+    )
+
+    fig = plt.figure()
+    ax = fig.add_subplot(111)
+
+    times = [datetime.datetime(2021, 1, 1)]
+    while len(times) < 7:
+        times.append(times[-1] + datetime.timedelta(seconds=120))
+
+    y_vals = np.arange(5)
+
+    time_axis, y_axis = np.meshgrid(times, y_vals)
+    shape = (len(y_vals) - 1, len(times) - 1)
+    z_data = np.arange(shape[0] * shape[1])
+
+    z_data.shape = shape
+    try:
+        register_matplotlib_converters()
+
+        im = ax.pcolormesh(time_axis, y_axis, z_data)
+        # make sure this does not raise!
+        fig.canvas.draw()
+    finally:
+        deregister_matplotlib_converters()
+
+
 def test_bar_pandas(pd):
     # Smoke test for pandas
     df = pd.DataFrame(