 on the data point.
  // Its opacity is set to 0: we don't see it by default.
  const tooltip = d3.select("#my_dataviz")
    .append("div")
    .style("opacity", 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 d