Function render_sparkline 

pub fn render_sparkline(
    kind: SparklineKind,
    values: &[Option<f64>],
    color: Rgba,
    markers: bool,
    w: f32,
    h: f32,
) -> Vec<Prim>

Draws a sparkline into a w×h box at the origin. Gaps (None) break lines; column and win/loss sparklines draw negatives in our “down” red.