Fix step plots (#4508)

Author: dcherian

doc/whats-new.rst

@@ -36,6 +36,7 @@ New Features
 Bug fixes
 ~~~~~~~~~
 
+- Fix :py:meth:`DataArray.plot.step`. By `Deepak Cherian <https://github.com/dcherian>`_.
 - Fix bug where reading a scalar value from a NetCDF file opened with the ``h5netcdf`` backend would raise a ``ValueError`` when ``decode_cf=True`` (:issue:`4471`, :pull:`4485`).
   By `Gerrit Holl <https://github.com/gerritholl>`_.
 - Fix bug where datetime64 times are silently changed to incorrect values if they are outside the valid date range for ns precision when provided in some other units (:issue:`4427`, :pull:`4454`).

xarray/plot/utils.py

@@ -513,16 +513,20 @@ def _resolve_intervals_1dplot(xval, yval, xlabel, ylabel, kwargs):
     # Is it a step plot? (see matplotlib.Axes.step)
     if kwargs.get("drawstyle", "").startswith("steps-"):
 
+        remove_drawstyle = False
         # Convert intervals to double points
         if _valid_other_type(np.array([xval, yval]), [pd.Interval]):
             raise TypeError("Can't step plot intervals against intervals.")
         if _valid_other_type(xval, [pd.Interval]):
             xval, yval = _interval_to_double_bound_points(xval, yval)
+            remove_drawstyle = True
         if _valid_other_type(yval, [pd.Interval]):
             yval, xval = _interval_to_double_bound_points(yval, xval)
+            remove_drawstyle = True
 
         # Remove steps-* to be sure that matplotlib is not confused
-        del kwargs["drawstyle"]
+        if remove_drawstyle:
+            del kwargs["drawstyle"]
 
     # Is it another kind of plot?
     else:

xarray/tests/test_plot.py

@@ -670,11 +670,13 @@ def setUp(self):
         self.darray = DataArray(easy_array((2, 3, 4)))
 
     def test_step(self):
-        self.darray[0, 0].plot.step()
+        hdl = self.darray[0, 0].plot.step()
+        assert "steps" in hdl[0].get_drawstyle()
 
-    @pytest.mark.parametrize("ds", ["pre", "post", "mid"])
-    def test_step_with_drawstyle(self, ds):
-        self.darray[0, 0].plot.step(drawstyle=ds)
+    @pytest.mark.parametrize("where", ["pre", "post", "mid"])
+    def test_step_with_where(self, where):
+        hdl = self.darray[0, 0].plot.step(where=where)
+        assert hdl[0].get_drawstyle() == f"steps-{where}"
 
     def test_coord_with_interval_step(self):
         """Test step plot with intervals."""