Plot of influence in regression. Plots studentized resids vs. leverage.
Parameters:  results : results instance
external : bool
alpha : float
criterion : str {‘DFFITS’, ‘Cooks’}
size : float
plot_alpha : float
ax : matplotlib Axes instance


Returns:  fig : matplotlib figure

Notes
Row labels for the observations in which the leverage, measured by the diagonal of the hat matrix, is high or the residuals are large, as the combination of large residuals and a high influence value indicates an influence point. The value of large residuals can be controlled using the alpha parameter. Large leverage points are identified as hat_i > 2 * (df_model + 1)/nobs.