ENH: option for groupby.hist to match bins

doc/source/whatsnew/v0.24.0.txt

@@ -638,6 +638,7 @@ 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
@@ -578,15 +579,27 @@ def wrapper(*args, **kwargs):
  # a little trickery for aggregation functions that need an axis
  # argument
  kwargs_with_axis = kwargs.copy()
+ kwargs_wo_axis = kwargs.copy()
+
  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_wo_axis.pop('equal_bins', False):
+ # GH-22222
+ # if bins==None, use default value used in `hist_series`
+ bins = kwargs_wo_axis.pop('bins', 10)
+ 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_wo_axis['bins'] = bins
+
  def curried_with_axis(x):
  return f(x, *args, **kwargs_with_axis)
 
  def curried(x):
- return f(x, *args, **kwargs)
+ return f(x, *args, **kwargs_wo_axis)
 
  # preserve the name so we can detect it when calling plot methods,
  # to avoid duplicates

pandas/plotting/_core.py

@@ -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
 
@@ -2480,6 +2483,7 @@ def hist_series(self, by=None, ax=None, grid=True, xlabelsize=None,
  matplotlib.axes.Axes.hist : Plot a histogram using matplotlib.
 
  """
+ # TODO: separate docstrings of series and groupby hist functions (GH-22241)
  import matplotlib.pyplot as plt
 
  if by is None:

pandas/tests/plotting/test_groupby.py

@@ -12,6 +12,23 @@
 from pandas.tests.plotting.common import TestPlotBase
 
 
+@td.skip_if_no_mpl
+def test_hist_bins_match():
+ # GH-22222
+ N = 100
+ bins = 5
+
+ np.random.seed(0)
+ df = DataFrame(np.append(np.random.randn(N), np.random.randn(N) / 10),
+ columns=['rand'])
+ df['group'] = [0] * N + [1] * N
+ g = df.groupby('group')['rand']
+ ax = g.hist(bins=bins, alpha=0.7, equal_bins=True)[0]
+ bin_width_group0 = ax.patches[0].get_width()
+ bin_width_group1 = ax.patches[bins].get_width()
+ assert np.isclose(bin_width_group0, bin_width_group1)
+
+
 @td.skip_if_no_mpl
 class TestDataFrameGroupByPlots(TestPlotBase):