text: /* eslint no-magic-numbers: 0 */
  /*
   * Returns an Epanechnikov (parabolic) kernel function which takes a
   * free smoothing parameter as input.
   * code from https://gist.github.com/mbostock/4341954
   * kernel info https://en.wikipedia.org/wiki/Kernel_(statistics)
   */
  export default function kernelEpanechnikov(smoothing = 5) {
    return val => (Math.abs(val / smoothing) <= 1 ? 0.75 * ((1 - val * val) / smoothing) : 0);
  }
  