Function convergence_svg 

pub fn convergence_svg(
    bests: &[f64],
    width: u32,
    height: u32,
    title: &str,
    y_axis_label: &str,
    _direction_minimize: bool,
) -> String

Render a per-generation “best fitness so far” trace as an SVG line plot. bests[i] is the best fitness after generation i.

direction_minimize controls which way is “improvement”: true for minimize problems, false for maximize.