demo.js

demo.js

tried like 
//update data
    rects.attr({
        "x": function (d) { return xScale(d.country); },
        "y": function (d) { return yScale(d.gdp); },
        "width": xScale.rangeBand(),
    }).
    attr("height", 0).
    attr("y", h).
    transition().duration(3000).
    delay(function (d, i) {
        return i * 200;
    }).
    attr("height", function (d) {
            return h - yScale(d.gdp)
        });
its working but bar is going downward with transition