Function plot_fit 

pub fn plot_fit(
    res: &LinearResults,
    exog: &Array2<f64>,
    exog_idx: usize,
) -> Figure

A plot_fit-style diagnostic: the observed response and the fitted values plotted against one regressor (column exog_idx of the design).

Returns the rendered Figure. Pure plotting helper — the fitted values it draws are taken straight from res.fittedvalues.