Function gapped_slice 

pub fn gapped_slice(
    center: Point,
    inner: f32,
    outer: f32,
    from: f32,
    to: f32,
    gap: f32,
) -> Vec<PathSegment>

A slice as slice_path draws it, with gap left clear all around it: half on each of its sides, of the same width all the way, and half at its ends. What is behind the chart shows through.