Function lineplot_add_slope 

pub fn lineplot_add_slope(
    plot: &mut Plot<GridCanvas>,
    intercept: f64,
    slope: f64,
    options: LineplotSeriesOptions,
) -> Result<(), LineplotError>

Adds a slope/intercept line to an existing plot.

The line is defined as y = intercept + slope * x across the plot’s current x-axis limits.

Errors

Returns LineplotError::InvalidNumericValue for non-finite values.