code: function makeFloorClassifier(nbClusters, arr) {
    var min = Math.min.apply(Math, arr);
    var delta = Math.max.apply(Math, arr) - min;
    min -= delta / nbClusters / 2;
    return function classify(value) {
      return Math.floor((nbClusters * (value - min)) / delta);
    };
  }
docs: makeClassifier(): makes a classifier, based on an array of numbers and an expected number of clusters.
  nbClusters: expected number of clusters
  arr: array of numbers
  => returns a function that takes a number, and returns the number of its corresponding column.