Function paint_diagnostic_underline 

pub fn paint_diagnostic_underline(
    scene: &mut Scene,
    transform: Affine,
    x0: f64,
    x1: f64,
    y: f64,
    color: AlphaColor<Srgb>,
)

A wavy underline from x0 to x1 along y: the mark of a problem in text, as spelling marks one.