atapoint (d)
  const showTooltip = function(event,d) {
    tooltip
      .transition()
      .duration(100)
      .style("opacity", 1)
    tooltip
      .html("Range: " + d.x0 + " - " + d.x1)
      .style("left", (event.x)/2-100 + "px")
      .style("top", (event.y)/2 + "px")
  }
  const moveTooltip = function(event,d) {
    tooltip
    .style("left", (event.x)/2-100 + "px")
    .style("top", (event.y)/2 + "px")
  }
  // A function that change this tooltip when the leaves a point: just need to set opacity t