Fix pcolormesh with str coords (#7612)

* pcolormesh with str coords

* add whats-new

doc/whats-new.rst

@@ -59,6 +59,8 @@ Bug fixes
   By `Jimmy Westling <https://github.com/illviljan>`_.
 - Improved performance in ``open_dataset`` for datasets with large object arrays (:issue:`7484`, :pull:`7494`).
   By `Alex Goodman <https://github.com/agoodm>`_ and `Deepak Cherian <https://github.com/dcherian>`_.
+- Fix :py:meth:`DataArray.plot.pcolormesh` which now works if one of the coordinates has str dtype  (:issue:`6775`, :pull:`7612`).
+  By `Michael Niklas <https://github.com/headtr1ck>`_.
 
 Documentation
 ~~~~~~~~~~~~~

xarray/plot/dataarray_plot.py

@@ -2293,27 +2293,23 @@ def pcolormesh(
         else:
             infer_intervals = True
 
-    if (
-        infer_intervals
-        and not np.issubdtype(x.dtype, str)
-        and (
-            (np.shape(x)[0] == np.shape(z)[1])
-            or ((x.ndim > 1) and (np.shape(x)[1] == np.shape(z)[1]))
-        )
+    if any(np.issubdtype(k.dtype, str) for k in (x, y)):
+        # do not infer intervals if any axis contains str ticks, see #6775
+        infer_intervals = False
+
+    if infer_intervals and (
+        (np.shape(x)[0] == np.shape(z)[1])
+        or ((x.ndim > 1) and (np.shape(x)[1] == np.shape(z)[1]))
     ):
-        if len(x.shape) == 1:
+        if x.ndim == 1:
             x = _infer_interval_breaks(x, check_monotonic=True, scale=xscale)
         else:
             # we have to infer the intervals on both axes
             x = _infer_interval_breaks(x, axis=1, scale=xscale)
             x = _infer_interval_breaks(x, axis=0, scale=xscale)
 
-    if (
-        infer_intervals
-        and not np.issubdtype(y.dtype, str)
-        and (np.shape(y)[0] == np.shape(z)[0])
-    ):
-        if len(y.shape) == 1:
+    if infer_intervals and (np.shape(y)[0] == np.shape(z)[0]):
+        if y.ndim == 1:
             y = _infer_interval_breaks(y, check_monotonic=True, scale=yscale)
         else:
             # we have to infer the intervals on both axes

xarray/tests/test_plot.py

@@ -427,6 +427,16 @@ def test2d_1d_2d_coordinates_pcolormesh(self) -> None:
             _, unique_counts = np.unique(v[:-1], axis=0, return_counts=True)
             assert np.all(unique_counts == 1)
 
+    def test_str_coordinates_pcolormesh(self) -> None:
+        # test for #6775
+        x = DataArray(
+            [[1, 2, 3], [4, 5, 6]],
+            dims=("a", "b"),
+            coords={"a": [1, 2], "b": ["a", "b", "c"]},
+        )
+        x.plot.pcolormesh()
+        x.T.plot.pcolormesh()
+
     def test_contourf_cmap_set(self) -> None:
         a = DataArray(easy_array((4, 4)), dims=["z", "time"])