Edit in JSFiddle

var dataSrc2 = new DataSrc();
var myCustomBarChart = BarChart();
var fadeOut = function() {
  this.attr("opacity", function(d, i) {
    return i / dataSrc2.data.length;
  });
};
myCustomBarChart.on("enter", fadeOut);
myCustomBarChart.on("update:transition", fadeOut);
myCustomBarChart.draw(dataSrc2);

d3.select("button").on("click", function() {
  setInterval(function() {
    dataSrc2.fetch();
    myCustomBarChart.draw(dataSrc2);
  }, 1500);
});

External resources loaded into this fiddle: