each specie
d3.csv("https://raw.githubusercontent.com/holtzy/D3-graph-gallery/master/DATA/iris.csv", function(data) {

  // Compute quartiles, median, inter quantile range min and max --> these info are then used to draw the box.
  var sumstat = d3.nest() // nest function allows to group the calculation per level of a factor
    .key(function(d) { return d.Species;})
    .rollup(function(d) {
      q1 = d3.quantile(d.map(function(g) { return g.Sepal_Length;}).sort(d3.ascending),.25)
      median = d3.quanti