interpolate.js

function interpolate(interpolation_data, new_value) {
  var keys = Object.keys(interpolation_data),
      min_value = Math.min.apply(Math, keys),
      max_value = Math.max.apply(Math, keys),
      min_image = interpolation_data[min_value],
      max_image = interpolation_data[max_value];
  return min_image + (max_image - min_image) * (new_value - min_value) / (max_value - min_value);
}

// when level = level_max, we get max PI
console.assert( interpolate({0: 2403, 50: 3206}, 50) === 3206 );
// at half level, we are half-way to the max PI
console.assert( interpolate({0: 2403, 50: 3206}, 25) === 2403 + (3206-2403)/2 );
// at level 1, we are at 1/50th of the way
console.assert( interpolate({0: 2403, 50: 3206},  1) === 2403 + (3206-2403)/50 );