Function truncate_to_width 

pub fn truncate_to_width(
    label: &str,
    max_width: f64,
    size: f64,
) -> Option<String>

Shorten label with a trailing ellipsis (“…”) until it (plus the ellipsis) fits within max_width px at font size (#159: a vertical multi-column legend grid that still overflows after exhausting column reduction falls back to truncating its labels). None means label already fits — no truncation needed, so the caller keeps the byte-identical untruncated path. A max_width too narrow for even a bare “…” still returns Some("…") — the best any truncation can do — rather than an empty string.