Allow binned coordinates on 1D plots y-axis. (#3685)

* Allow binned coords on 1D plots y-axis. See #3571.

* Add helper to convert intervals for 1d plots.

* Add tests for line plots with x/y/x&y intervals.

* Add tests for step plots with x/y intervals.

* Simplify syntax in 1D intervals plot helper.

* Document new handling of y-axis intervals.

Author: juseg

doc/whats-new.rst

@@ -102,6 +102,9 @@ Bug fixes
   By `Justus Magin <https://github.com/keewis>`_.
 - Fixed errors emitted by ``mypy --strict`` in modules that import xarray.
   (:issue:`3695`) by `Guido Imperiale <https://github.com/crusaderky>`_.
+- Fix plotting of binned coordinates on the y axis in :py:meth:`DataArray.plot`
+  (line) and :py:meth:`DataArray.plot.step` plots (:issue:`#3571`,
+  :pull:`3685`) by `Julien Seguinot <https://github.com/juseg>_`.
 
 Documentation
 ~~~~~~~~~~~~~

xarray/plot/plot.py

@@ -17,13 +17,11 @@
     _ensure_plottable,
     _infer_interval_breaks,
     _infer_xy_labels,
-    _interval_to_double_bound_points,
-    _interval_to_mid_points,
     _process_cmap_cbar_kwargs,
     _rescale_imshow_rgb,
+    _resolve_intervals_1dplot,
     _resolve_intervals_2dplot,
     _update_axes,
-    _valid_other_type,
     get_axis,
     import_matplotlib_pyplot,
     label_from_attrs,
@@ -296,29 +294,10 @@ def line(
     ax = get_axis(figsize, size, aspect, ax)
     xplt, yplt, hueplt, xlabel, ylabel, hue_label = _infer_line_data(darray, x, y, hue)
 
-    # Remove pd.Intervals if contained in xplt.values.
-    if _valid_other_type(xplt.values, [pd.Interval]):
-        # Is it a step plot? (see matplotlib.Axes.step)
-        if kwargs.get("linestyle", "").startswith("steps-"):
-            xplt_val, yplt_val = _interval_to_double_bound_points(
-                xplt.values, yplt.values
-            )
-            # Remove steps-* to be sure that matplotlib is not confused
-            kwargs["linestyle"] = (
-                kwargs["linestyle"]
-                .replace("steps-pre", "")
-                .replace("steps-post", "")
-                .replace("steps-mid", "")
-            )
-            if kwargs["linestyle"] == "":
-                del kwargs["linestyle"]
-        else:
-            xplt_val = _interval_to_mid_points(xplt.values)
-            yplt_val = yplt.values
-            xlabel += "_center"
-    else:
-        xplt_val = xplt.values
-        yplt_val = yplt.values
+    # Remove pd.Intervals if contained in xplt.values and/or yplt.values.
+    xplt_val, yplt_val, xlabel, ylabel, kwargs = _resolve_intervals_1dplot(
+        xplt.values, yplt.values, xlabel, ylabel, kwargs
+    )
 
     _ensure_plottable(xplt_val, yplt_val)
 
@@ -367,7 +346,7 @@ def step(darray, *args, where="pre", linestyle=None, ls=None, **kwargs):
           every *x* position, i.e. the interval ``[x[i], x[i+1])`` has the
           value ``y[i]``.
         - 'mid': Steps occur half-way between the *x* positions.
-        Note that this parameter is ignored if the x coordinate consists of
+        Note that this parameter is ignored if one coordinate consists of
         :py:func:`pandas.Interval` values, e.g. as a result of
         :py:func:`xarray.Dataset.groupby_bins`. In this case, the actual
         boundaries of the interval are used.

xarray/plot/utils.py

@@ -453,6 +453,42 @@ def _interval_to_double_bound_points(xarray, yarray):
     return xarray, yarray
 
 
+def _resolve_intervals_1dplot(xval, yval, xlabel, ylabel, kwargs):
+    """
+    Helper function to replace the values of x and/or y coordinate arrays
+    containing pd.Interval with their mid-points or - for step plots - double
+    points which double the length.
+    """
+
+    # Is it a step plot? (see matplotlib.Axes.step)
+    if kwargs.get("linestyle", "").startswith("steps-"):
+
+        # 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)
+        if _valid_other_type(yval, [pd.Interval]):
+            yval, xval = _interval_to_double_bound_points(yval, xval)
+
+        # Remove steps-* to be sure that matplotlib is not confused
+        del kwargs["linestyle"]
+
+    # Is it another kind of plot?
+    else:
+
+        # Convert intervals to mid points and adjust labels
+        if _valid_other_type(xval, [pd.Interval]):
+            xval = _interval_to_mid_points(xval)
+            xlabel += "_center"
+        if _valid_other_type(yval, [pd.Interval]):
+            yval = _interval_to_mid_points(yval)
+            ylabel += "_center"
+
+    # return converted arguments
+    return xval, yval, xlabel, ylabel, kwargs
+
+
 def _resolve_intervals_2dplot(val, func_name):
     """
     Helper function to replace the values of a coordinate array containing

xarray/tests/test_plot.py

@@ -426,9 +426,25 @@ def test_convenient_facetgrid_4d(self):
             d.plot(x="x", y="y", col="columns", ax=plt.gca())
 
     def test_coord_with_interval(self):
+        """Test line plot with intervals."""
         bins = [-1, 0, 1, 2]
         self.darray.groupby_bins("dim_0", bins).mean(...).plot()
 
+    def test_coord_with_interval_x(self):
+        """Test line plot with intervals explicitly on x axis."""
+        bins = [-1, 0, 1, 2]
+        self.darray.groupby_bins("dim_0", bins).mean(...).plot(x="dim_0_bins")
+
+    def test_coord_with_interval_y(self):
+        """Test line plot with intervals explicitly on y axis."""
+        bins = [-1, 0, 1, 2]
+        self.darray.groupby_bins("dim_0", bins).mean(...).plot(y="dim_0_bins")
+
+    def test_coord_with_interval_xy(self):
+        """Test line plot with intervals on both x and y axes."""
+        bins = [-1, 0, 1, 2]
+        self.darray.groupby_bins("dim_0", bins).mean(...).dim_0_bins.plot()
+
 
 class TestPlot1D(PlotTestCase):
     @pytest.fixture(autouse=True)
@@ -511,10 +527,23 @@ def test_step(self):
         self.darray[0, 0].plot.step()
 
     def test_coord_with_interval_step(self):
+        """Test step plot with intervals."""
         bins = [-1, 0, 1, 2]
         self.darray.groupby_bins("dim_0", bins).mean(...).plot.step()
         assert len(plt.gca().lines[0].get_xdata()) == ((len(bins) - 1) * 2)
 
+    def test_coord_with_interval_step_x(self):
+        """Test step plot with intervals explicitly on x axis."""
+        bins = [-1, 0, 1, 2]
+        self.darray.groupby_bins("dim_0", bins).mean(...).plot.step(x="dim_0_bins")
+        assert len(plt.gca().lines[0].get_xdata()) == ((len(bins) - 1) * 2)
+
+    def test_coord_with_interval_step_y(self):
+        """Test step plot with intervals explicitly on y axis."""
+        bins = [-1, 0, 1, 2]
+        self.darray.groupby_bins("dim_0", bins).mean(...).plot.step(y="dim_0_bins")
+        assert len(plt.gca().lines[0].get_xdata()) == ((len(bins) - 1) * 2)
+
 
 class TestPlotHistogram(PlotTestCase):
     @pytest.fixture(autouse=True)