Function hairline 

pub fn hairline(alpha: f32) -> Hsla

Translucent hairline ink for borders, dividers and rings: a light grey on dark, a dark grey on light at INK_HAIRLINE_SCALE of the alpha. Never pure white or black.

Separate from ink because edges and fills scale in opposite directions when the field brightens — a 1px line needs more ink on white, a plate needs less.