ENH: option for groupby.hist to match bins

doc/source/whatsnew/v0.24.0.txt

@@ -640,6 +640,7 @@ Plotting
 
 - Bug in :func:`DataFrame.plot.scatter` and :func:`DataFrame.plot.hexbin` caused x-axis label and ticklabels to disappear when colorbar was on in IPython inline backend (:issue:`10611`, :issue:`10678`, and :issue:`20455`)
 - Bug in plotting a Series with datetimes using :func:`matplotlib.axes.Axes.scatter` (:issue:`22039`)
+- The new argument ``equal_bins`` in :func:`DataFrameGroupBy.hist` sets histogram bins to equal values (:issue:`22222`)
 
 Groupby/Resample/Rolling
 ^^^^^^^^^^^^^^^^^^^^^^^^

pandas/core/groupby/groupby.py

@@ -28,6 +28,7 @@ class providing the base-class of operations.
 from pandas.core.dtypes.common import (
  is_numeric_dtype,
  is_scalar,
+ is_integer,
  ensure_float)
 from pandas.core.dtypes.cast import maybe_downcast_to_dtype
 from pandas.core.dtypes.missing import isna, notna
@@ -581,6 +582,17 @@ def wrapper(*args, **kwargs):
  if 'axis' not in kwargs_with_axis or \
  kwargs_with_axis['axis'] is None:
  kwargs_with_axis['axis'] = self.axis
+ if (name == 'hist' and
+ kwargs.get('equal_bins', False) is True):
+ # GH-22222
+ bins = kwargs.get('bins')
+ if bins is None:
+ bins = 10 # use default value used in `hist_series`
+ if is_integer(bins):
+ # share the same numpy array for all group bins
+ bins = np.linspace(self.obj.min(),
+ self.obj.max(), bins + 1)
+ kwargs['bins'] = bins
 
  def curried_with_axis(x):
  return f(x, *args, **kwargs_with_axis)

pandas/plotting/_core.py

@@ -2443,7 +2443,7 @@ def hist_frame(data, column=None, by=None, grid=True, xlabelsize=None,
 
 def hist_series(self, by=None, ax=None, grid=True, xlabelsize=None,
  xrot=None, ylabelsize=None, yrot=None, figsize=None,
- bins=10, **kwds):
+ bins=10, equal_bins=False, **kwds):
  """
  Draw histogram of the input series using matplotlib
 
@@ -2470,8 +2470,11 @@ def hist_series(self, by=None, ax=None, grid=True, xlabelsize=None,
  bin edges are calculated and returned. If bins is a sequence, gives
  bin edges, including left edge of first bin and right edge of last
  bin. In this case, bins is returned unmodified.
- bins: integer, default 10
- Number of histogram bins to be used
+ equal_bins : boolean, default False
+ Uses the overall maximum and minimum of the groups to set a shared
+ bins sequence, leading to equal bin widths for all
+ groups (only works if bins==None or int).
+
  `**kwds` : keywords
  To be passed to the actual plotting function
 

pandas/tests/plotting/test_groupby.py

@@ -10,10 +10,54 @@
 import numpy as np
 
 from pandas.tests.plotting.common import TestPlotBase
+import pytest
 
 
 @td.skip_if_no_mpl
 class TestDataFrameGroupByPlots(TestPlotBase):
+ @pytest.mark.parametrize(
+ 'bins, equal_bins',
+ zip([5, None, np.linspace(-3, 3, 10)], [True, False])
+ )
+ def test_hist_bins_match(self, bins, equal_bins):
+ # GH-22222
+ N = 100
+ df = DataFrame(np.append(np.linspace(-3, 3, N),
+ np.linspace(-1, 1, N)),
+ columns=['rand'])
+ df['group'] = [0] * N + [1] * N
+ g = df.groupby('group')['rand']
+ axes = g.hist(bins=bins, alpha=0.7, equal_bins=equal_bins)
+ ax = axes[0]
+
+ if bins is None:
+ num_bins = 10 # default value used in `hist_series`
+ elif type(bins) == np.ndarray:
+ num_bins = len(bins) - 1
+ else:
+ num_bins = bins
+
+ # individual hist bar coordinates for:
+ # group0: points[:num_bins]
+ # group1: points[num_bins:]
+ points = np.array([patch.get_bbox().get_points()
+ for patch in ax.patches])
+
+ if equal_bins or type(bins) == np.ndarray:
+ # compare leftmost point on x-axis for each bar
+ # in the two groups
+ assert np.isclose(points[:num_bins, 0, 0],
+ points[num_bins:, 0, 0]
+ ).all()
+ else:
+ # range of values on x-axis (min, max) for each group
+ hist_ranges = DataFrame(
+ [[points[0, 0, 0], points[num_bins - 1, 1, 0]],
+ [points[num_bins, 0, 0], points[-1, 1, 0]]],
+ index=[0, 1], columns=['min', 'max'])
+ group_ranges = g.agg([min, max])
+ assert np.isclose(group_ranges, hist_ranges).all()
+ tm.close()
 
  def test_series_groupby_plotting_nominally_works(self):
  n = 10