code: def plot_alpha_returns(alpha_returns, ax=None):
      
      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
docs: 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