Chart.js/src/helpers/helpers.interpolation.js

/**
 * @private
 */
export function _pointInLine(p1, p2, t, mode) { // eslint-disable-line no-unused-vars
  return {
    x: p1.x + t * (p2.x - p1.x),
    y: p1.y + t * (p2.y - p1.y)
  };
}

/**
 * @private
 */
export function _steppedInterpolation(p1, p2, t, mode) {
  return {
    x: p1.x + t * (p2.x - p1.x),
    y: mode === 'middle' ? t < 0.5 ? p1.y : p2.y
    : mode === 'after' ? t < 1 ? p1.y : p2.y
    : t > 0 ? p2.y : p1.y
  };
}

/**
 * @private
 */
export function _bezierInterpolation(p1, p2, t, mode) { // eslint-disable-line no-unused-vars
  const cp1 = {x: p1.controlPointNextX, y: p1.controlPointNextY};
  const cp2 = {x: p2.controlPointPreviousX, y: p2.controlPointPreviousY};
  const a = _pointInLine(p1, cp1, t);
  const b = _pointInLine(cp1, cp2, t);
  const c = _pointInLine(cp2, p2, t);
  const d = _pointInLine(a, b, t);
  const e = _pointInLine(b, c, t);
  return _pointInLine(d, e, t);
}