a);

  // Y axis: scale and draw:
  var y = d3.scaleLinear()
      .range([height, 0]);
      y.domain([0, d3.max(bins, function(d) { return d.length; })]);   // d3.hist has to be called before the Y axis obviously
  svg.append("g")
      .call(d3.axisLeft(y));

  // Add a tooltip div. Here I define the general feature of the tooltip: stuff that do not depend on the data point.
  // Its opacity is set to 0: we don't see it by default.
  var tooltip = d3.select("#my_dataviz")
    .append("div")
    .style("o