Function measure_text 

pub fn measure_text(text: &str, metrics: &TextMetrics) -> f64

Measure the rendered width of a text string under the given metrics.

This is the self-correcting text width estimator used by every layout decision: margins, tick strategy, legend packing, truncation, and label rotation. It accounts for:

• font_size_px — linear scaling from the 12px calibration baseline.
• text_transform — transforms the text before measuring so uppercase labels are measured as uppercase (wider than the original).
• letter_spacing_px — CSS letter-spacing adds a constant per glyph.
• monospace — monospace faces are measured with a fixed per-character advance that is wider than the sans-serif default.

Backward compatibility: when metrics.is_legacy_default() is true, the result is identical (to the bit) to approximate_text_width(text). This is the invariant that keeps the pre-theme-hooks golden snapshots byte-identical under Theme::default().