var timer;

    function handleClick(d, i) {

      d3.select(this).classed("active", !d3.select(this).classed("active"));

    }

    function handleClickAll() {

      d3.selectAll(".bar")

        .classed("active", function (d, i) {

          return !d3.select(this).classed("active");

        });

    }

    function handleStart() {

      timer = setInterval(handleClickAll, 500);

    }

    function handleStop() {

      clearInterval(timer);

    }

    d3.selectAll(".bar")

      .on("click", handleClick);

    document.getElementById("all").addEventListener("click", handleClickAll);

    document.getElementById("start").addEventListener("click", handleStart);

    document.getElementById("stop").addEventListener("click", handleStop);