d the bar rectangles to the svg element
  svg.selectAll("rect")
      .data(bins)
      .enter()
      .append("rect")
        .attr("x", 1)
        .attr("transform", function(d) { return "translate(" + x(d.x0) + "," + y(d.length) + ")"; })
        .attr("width", function(d) { return x(d.x1) - x(d.x0) -1 ; })
        .attr("height", function(d) { return height - y(d.length); })
        .style("fill", "#69b3a2")
        // Show tooltip on hover
        .on("mouseover", showTooltip )
        .on("mousemove",