docs: def predictions_variance(df, filepath=None):
      """
      Plots the mean variance prediction for each readout
  
      Parameters
      ----------
      df: `pandas.DataFrame`_
          DataFrame with columns starting with `VAR:`
  
      filepath: str
          Absolute path to a folder where to write the plots
  
  
      Returns
      -------
      plot
          Generated plot
  
  
      .. _pandas.DataFrame: http://pandas.pydata.org/pandas-docs/stable/dsintro.html#dataframe
      """
  
      df = df.filter(regex="^VAR:")
  
      by_readout = df.mean(axis=0).reset_index(level=0)
      by_readout.columns = ['Readout', 'Prediction variance (mean)']
  
      by_readout['Readout'] = by_readout.Readout.map(lambda n: n[4:])
  
      g1 = sns.factorplot(x='Readout', y='Prediction variance (mean)', data=by_readout, kind='bar', aspect=2)
  
      for tick in g1.ax.get_xticklabels():
          tick.set_rotation(90)
  
      if filepath:
          g1.savefig(os.path.join(filepath, 'predictions-variance.pdf'))
  
      return g1