v4.22.0 — Configurable series start extension

Added

• Time-varying thresholds and Form-B reference lines now support
  extendToStart: false, which leaves the chart unchanged before the series'
  first point instead of carrying that first value backward to the visible
  window start. Threshold stroke coloring, fill, marker, badge, and range fitting
  all honor the cutoff. Resolves
  #326.