code: def plot_alpha_returns(alpha_returns, ax=None):
      """
      Plot histogram of daily multi-factor alpha returns (specific returns).
  
      Parameters
      ----------
      alpha_returns : pd.Series
          series of daily alpha returns indexed by datetime
  
      ax :  matplotlib.axes.Axes
          axes on which plots are made. if None, current axes will be used
  
      Returns
      -------
      ax :  matplotlib.axes.Axes
      """
      if ax is None:
          ax = plt.gca()
  
      ax.hist(alpha_returns, color="g", label="Multi-factor alpha")
      ax.set_title("Histogram of alphas")
      ax.axvline(0, color="k", linestyle="--", label="Zero")
  
      avg = alpha_returns.mean()
      ax.axvline(avg, color="b", label="Mean = {: 0.5f}".format(avg))
      configure_legend(ax)
  
      return ax
  
file_path: pyfolio/pyfolio/perf_attrib.py
imports: from __future__ import division
  import warnings
  from collections import OrderedDict
  import empyrical
  import pandas
  import matplotlib.pyplot
  from .pos import get_percent_alloc
  from .txn import get_turnover
  from .utils import print_table, configure_legend
label_desc: Use image node_id 6 for calling a global function with example usage: plot_alpha_returns(alpha_returns, ax) and returns: ax