scaled-interpolation.js

const map = [
  [4690, 0],
  [4648, 20.0],
  [4480, 30.0],
  [4340, 45.0],
  [4172, 60.0],
  [3976, 80.0],
  [3752, 100.0],
  [3276, 150.0],
  [2772, 200.0],
  [2268, 250.0],
  [2100, 270.0],
  [1876, 290.0],
  [1736, 305.0],
  [1596, 320.0],
  [1484, 330.0],
  [1372, 340.0],
  [1260, 350.0],
  [1148, 360.0],
  [1092, 370.0],
  [1008, 380.0],
  [896, 390.0],
  [784, 400.0],
  [700, 410.0],
  [588, 420.0],
  [476, 430.0],
  [392, 440.0],
  [336, 445.0],
  [0, 450.0]
];

/**
 * Rescale value between 2 ranges
 *
 *        (dstMax-dstMin) * (x - srcMin)
 * f(x) = ------------------------------  + dstMin
 *              srcMax - srcMin
 */
const scale = (src, dst, val) => {
  return (dst[1] - dst[0]) * (val - src[0]) / (src[1] - src[0]) + dst[0];
}

const isBetween = (bounds, val) => Math.min(...bounds) <= val && val <= Math.max(...bounds)
const isDecrease = (a, b) => (a - b) < 0
const isIncrease = (a, b) => (a - b) > 0

const interpolate = (pts, val) => {
  if (!Array.isArray(pts)) {
    throw new TypeError('Points map must be type of array.')
  }
  const isOrdered = (pts[0][0] - pts[1][0]) > 0 ? isDecrease : isIncrease

  let prev = pts[0];
  for (let i = 1; i < pts.length; i += 1) {
    if (isOrdered(prev[0], pts[i][0])) {
      throw new Error('Points map must be ordered.');
    }
    if (isBetween([prev[0], pts[i][0]], val)) {
      return scale([prev[0], pts[i][0]], [prev[1], pts[i][1]], val)
    }
    prev = pts[i];
  }
  throw new RangeError(`Value ${val} is out of range ${pts[0][0]}..${prev[0]}`);
}

const pc = 68;
const mm = scale([0, 100], [map[0][0], map[map.length - 1][0]], pc);
const lt = interpolate(map, mm);

console.log(mm);
console.log(lt);