Function paint_gap 

pub fn paint_gap(
    painter: &Painter,
    area: Rect,
    controller: &Controller,
    reference: f64,
    palette: Palette,
) -> Option<Rect>

Paints each trial’s gap value − reference on a log scale, with the best-so-far gap as a step line.

Pruned and paused trials are hollow: their value is a partial report. Returns the plot rectangle, if it fits.