Function span_bar 

fn span_bar(
    s: &Yaml,
    depth: usize,
    sel: bool,
    ends: [usize; 4],
    t: &Trace,
) -> String

One span’s row: name, service, bar, duration, each clipped to its column.

ends is the column boundaries from App::waterfall. Every write caps the row at its own end before writing, so an over-long span name eats into its own column and nothing else — without the cap it would push the bar right and shove the duration off the screen.