pacity", 0)
    .attr("class", "tooltip")
    .style("background-color", "black")
    .style("color", "white")
    .style("border-radius", "5px")
    .style("padding", "10px")

  // A function that change this tooltip when the user hover a point.
  // Its opacity is set to 1: we can now see it. Plus it set the text and position of tooltip depending on the datapoint (d)
  var showTooltip = function(d) {
    tooltip
      .transition()
      .duration(100)
      .style("opacity", 1)
    tooltip
      .html("R