ENH: Plotting for groupby_bins

doc/plotting.rst

@@ -222,9 +222,41 @@ It is also possible to make line plots such that the data are on the x-axis and
     @savefig plotting_example_xy_kwarg.png
     air.isel(time=10, lon=[10, 11]).plot(y='lat', hue='lon')
 
+Step plots
+~~~~~~~~~~
+
+As an alternative, also a step plot similar to matplotlib's ``plt.step`` can be
+made using 1D data. 
+
+.. ipython:: python
+
+    @savefig plotting_example_step.png width=4in
+    air1d[:20].plot.step(where='mid')
+
+The argument ``where`` defines where the steps should be placed, options are
+``'pre'`` (default), ``'post'``, and ``'mid'``. This is particularly handy
+when plotting data grouped with :py:func:`xarray.Dataset.groupby_bins`.
+
+.. ipython:: python
+
+    air_grp = air.mean(['time','lon']).groupby_bins('lat',[0,23.5,66.5,90])
+    air_mean = air_grp.mean()
+    air_std = air_grp.std()
+    air_mean.plot.step()
+    (air_mean + air_std).plot.step(ls=':')
+    (air_mean - air_std).plot.step(ls=':')
+    plt.ylim(-20,30)
+    @savefig plotting_example_step_groupby.png width=4in
+    plt.title('Zonal mean temperature')
+
+In this case, the actual boundaries of the bins are used and the ``where`` argument
+is ignored.
+
+
 Other axes kwargs
 -----------------
 
+
 The keyword arguments ``xincrease`` and ``yincrease`` let you control the axes direction.
 
 .. ipython:: python

doc/whats-new.rst

@@ -59,6 +59,9 @@ Enhancements
 
 - Added support for Python 3.7. (:issue:`2271`).
   By `Joe Hamman <https://github.com/jhamman>`_.
+- Added support for plotting data with `pandas.Interval` coordinates, such as those
+  created by :py:meth:`~xarray.DataArray.groupby_bins`
+  By `Maximilian Maahn <https://github.com/maahn>`_.
 - Added :py:meth:`~xarray.CFTimeIndex.shift` for shifting the values of a
   CFTimeIndex by a specified frequency. (:issue:`2244`).
   By `Spencer Clark <https://github.com/spencerkclark>`_.

xarray/plot/__init__.py

@@ -1,14 +1,15 @@
 from __future__ import absolute_import
 from __future__ import division
 from __future__ import print_function
-from .plot import (plot, line, contourf, contour,
+from .plot import (plot, line, step, contourf, contour,
                    hist, imshow, pcolormesh)
 
 from .facetgrid import FacetGrid
 
 __all__ = [
     'plot',
     'line',
+    'step',
     'contour',
     'contourf',
     'hist',

xarray/plot/plot.py

@@ -19,7 +19,9 @@
 
 from .facetgrid import FacetGrid
 from .utils import (
-    ROBUST_PERCENTILE, _determine_cmap_params, _infer_xy_labels, get_axis,
+    ROBUST_PERCENTILE, _determine_cmap_params, _infer_xy_labels,
+    _interval_to_double_bound_points, _interval_to_mid_points,
+    _resolve_intervals_2dplot, _valid_other_type, get_axis,
     import_matplotlib_pyplot, label_from_attrs)
 
 
@@ -35,27 +37,20 @@ def _valid_numpy_subdtype(x, numpy_types):
     return any(np.issubdtype(x.dtype, t) for t in numpy_types)
 
 
-def _valid_other_type(x, types):
-    """
-    Do all elements of x have a type from types?
-    """
-    return all(any(isinstance(el, t) for t in types) for el in np.ravel(x))
-
-
 def _ensure_plottable(*args):
     """
     Raise exception if there is anything in args that can't be plotted on an
-    axis.
+    axis by matplotlib.
     """
     numpy_types = [np.floating, np.integer, np.timedelta64, np.datetime64]
     other_types = [datetime]
 
     for x in args:
-        if not (_valid_numpy_subdtype(np.array(x), numpy_types) or
-                _valid_other_type(np.array(x), other_types)):
+        if not (_valid_numpy_subdtype(np.array(x), numpy_types)
+                or _valid_other_type(np.array(x), other_types)):
             raise TypeError('Plotting requires coordinates to be numeric '
                             'or dates of type np.datetime64 or '
-                            'datetime.datetime.')
+                            'datetime.datetime or pd.Interval.')
 
 
 def _easy_facetgrid(darray, plotfunc, x, y, row=None, col=None,
@@ -326,9 +321,30 @@ def line(darray, *args, **kwargs):
     xplt, yplt, hueplt, xlabel, ylabel, huelabel = \
         _infer_line_data(darray, x, y, hue)
 
-    _ensure_plottable(xplt)
+    # 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'] == '':
+                kwargs.pop('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
 
-    primitive = ax.plot(xplt, yplt, *args, **kwargs)
+    _ensure_plottable(xplt_val, yplt_val)
+
+    primitive = ax.plot(xplt_val, yplt_val, *args, **kwargs)
 
     if _labels:
         if xlabel is not None:
@@ -359,6 +375,46 @@ def line(darray, *args, **kwargs):
     return primitive
 
 
+def step(darray, *args, **kwargs):
+    """
+    Step plot of DataArray index against values
+
+    Similar to :func:`matplotlib:matplotlib.pyplot.step`
+
+    Parameters
+    ----------
+    where : {'pre', 'post', 'mid'}, optional, default 'pre'
+        Define where the steps should be placed:
+        - 'pre': The y value is continued constantly to the left from
+          every *x* position, i.e. the interval ``(x[i-1], x[i]]`` has the
+          value ``y[i]``.
+        - 'post': The y value is continued constantly to the right from
+          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
+        :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.
+
+    *args, **kwargs : optional
+        Additional arguments following :py:func:`xarray.plot.line`
+
+    """
+    if ('ls' in kwargs.keys()) and ('linestyle' not in kwargs.keys()):
+        kwargs['linestyle'] = kwargs.pop('ls')
+
+    where = kwargs.pop('where', 'pre')
+
+    if where not in ('pre', 'post', 'mid'):
+        raise ValueError("'where' argument to step must be "
+                         "'pre', 'post' or 'mid'")
+
+    kwargs['linestyle'] = 'steps-' + where + kwargs.get('linestyle', '')
+
+    return line(darray, *args, **kwargs)
+
+
 def hist(darray, figsize=None, size=None, aspect=None, ax=None, **kwargs):
     """
     Histogram of DataArray
@@ -476,6 +532,10 @@ def hist(self, ax=None, **kwargs):
     def line(self, *args, **kwargs):
         return line(self._da, *args, **kwargs)
 
+    @functools.wraps(step)
+    def step(self, *args, **kwargs):
+        return step(self._da, *args, **kwargs)
+
 
 def _rescale_imshow_rgb(darray, vmin, vmax, robust):
     assert robust or vmin is not None or vmax is not None
@@ -716,7 +776,11 @@ def newplotfunc(darray, x=None, y=None, figsize=None, size=None,
         # Pass the data as a masked ndarray too
         zval = darray.to_masked_array(copy=False)
 
-        _ensure_plottable(xval, yval)
+        # Replace pd.Intervals if contained in xval or yval.
+        xplt, xlab_extra = _resolve_intervals_2dplot(xval, plotfunc.__name__)
+        yplt, ylab_extra = _resolve_intervals_2dplot(yval, plotfunc.__name__)
+
+        _ensure_plottable(xplt, yplt)
 
         if 'contour' in plotfunc.__name__ and levels is None:
             levels = 7  # this is the matplotlib default
@@ -756,16 +820,16 @@ def newplotfunc(darray, x=None, y=None, figsize=None, size=None,
                              "in xarray")
 
         ax = get_axis(figsize, size, aspect, ax)
-        primitive = plotfunc(xval, yval, zval, ax=ax, cmap=cmap_params['cmap'],
+        primitive = plotfunc(xplt, yplt, zval, ax=ax, cmap=cmap_params['cmap'],
                              vmin=cmap_params['vmin'],
                              vmax=cmap_params['vmax'],
                              norm=cmap_params['norm'],
                              **kwargs)
 
         # Label the plot with metadata
         if add_labels:
-            ax.set_xlabel(label_from_attrs(darray[xlab]))
-            ax.set_ylabel(label_from_attrs(darray[ylab]))
+            ax.set_xlabel(label_from_attrs(darray[xlab], xlab_extra))
+            ax.set_ylabel(label_from_attrs(darray[ylab], ylab_extra))
             ax.set_title(darray._title_for_slice())
 
         if add_colorbar:
@@ -794,7 +858,7 @@ def newplotfunc(darray, x=None, y=None, figsize=None, size=None,
         # Do this without calling autofmt_xdate so that x-axes ticks
         # on other subplots (if any) are not deleted.
         # https://stackoverflow.com/questions/17430105/autofmt-xdate-deletes-x-axis-labels-of-all-subplots
-        if np.issubdtype(xval.dtype, np.datetime64):
+        if np.issubdtype(xplt.dtype, np.datetime64):
             for xlabels in ax.get_xticklabels():
                 xlabels.set_rotation(30)
                 xlabels.set_ha('right')
@@ -995,14 +1059,22 @@ def pcolormesh(x, y, z, ax, infer_intervals=None, **kwargs):
         else:
             infer_intervals = True
 
-    if infer_intervals:
+    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:
             x = _infer_interval_breaks(x, check_monotonic=True)
-            y = _infer_interval_breaks(y, check_monotonic=True)
         else:
             # we have to infer the intervals on both axes
             x = _infer_interval_breaks(x, axis=1)
             x = _infer_interval_breaks(x, axis=0)
+
+    if (infer_intervals and
+            (np.shape(y)[0] == np.shape(z)[0])):
+        if len(y.shape) == 1:
+            y = _infer_interval_breaks(y, check_monotonic=True)
+        else:
+            # we have to infer the intervals on both axes
             y = _infer_interval_breaks(y, axis=1)
             y = _infer_interval_breaks(y, axis=0)
 

xarray/plot/utils.py

@@ -1,9 +1,11 @@
 from __future__ import absolute_import, division, print_function
 
+import itertools
 import textwrap
 import warnings
 
 import numpy as np
+import pandas as pd
 
 from ..core.options import OPTIONS
 from ..core.pycompat import basestring
@@ -367,7 +369,7 @@ def get_axis(figsize, size, aspect, ax):
     return ax
 
 
-def label_from_attrs(da):
+def label_from_attrs(da, extra=''):
     ''' Makes informative labels if variable metadata (attrs) follows
         CF conventions. '''
 
@@ -385,4 +387,66 @@ def label_from_attrs(da):
     else:
         units = ''
 
-    return '\n'.join(textwrap.wrap(name + units, 30))
+    return '\n'.join(textwrap.wrap(name + extra + units, 30))
+
+
+def _interval_to_mid_points(array):
+    """
+    Helper function which returns an array
+    with the Intervals' mid points.
+    """
+
+    return np.array([x.mid for x in array])
+
+
+def _interval_to_bound_points(array):
+    """
+    Helper function which returns an array
+    with the Intervals' boundaries.
+    """
+
+    array_boundaries = np.array([x.left for x in array])
+    array_boundaries = np.concatenate(
+        (array_boundaries, np.array([array[-1].right])))
+
+    return array_boundaries
+
+
+def _interval_to_double_bound_points(xarray, yarray):
+    """
+    Helper function to deal with a xarray consisting of pd.Intervals. Each
+    interval is replaced with both boundaries. I.e. the length of xarray
+    doubles. yarray is modified so it matches the new shape of xarray.
+    """
+
+    xarray1 = np.array([x.left for x in xarray])
+    xarray2 = np.array([x.right for x in xarray])
+
+    xarray = list(itertools.chain.from_iterable(zip(xarray1, xarray2)))
+    yarray = list(itertools.chain.from_iterable(zip(yarray, yarray)))
+
+    return xarray, yarray
+
+
+def _resolve_intervals_2dplot(val, func_name):
+    """
+    Helper function to replace the values of a coordinate array containing
+    pd.Interval with their mid-points or - for pcolormesh - boundaries which
+    increases length by 1.
+    """
+    label_extra = ''
+    if _valid_other_type(val, [pd.Interval]):
+        if func_name == 'pcolormesh':
+            val = _interval_to_bound_points(val)
+        else:
+            val = _interval_to_mid_points(val)
+            label_extra = '_center'
+
+    return val, label_extra
+
+
+def _valid_other_type(x, types):
+    """
+    Do all elements of x have a type from types?
+    """
+    return all(any(isinstance(el, t) for t in types) for el in np.ravel(x))