docs: def clscale (cl):
      """Say we take a Gaussian bivariate and convert the parameters of the
      distribution to an ellipse (major, minor, PA). By what factor should we
      scale those axes to make the area of the ellipse correspond to the
      confidence interval CL? (I.e. 0 < CL < 1)
  
      """
      rv = np.sqrt (-2 * np.log (1 - cl))
      rv[np.where (cl <= 0)] = np.nan
      return rv