Update D3 chart dynamically - d3.js

Question

The idea is to have a d3 vertical bar-chart that will be given live data.
I simulate the live data with a setInterval function that updates the the values of the elements in my dataset:
var updateData = function(){
a = parseInt(Math.random() * 100),
b = parseInt(Math.random() * 100),
c = parseInt(Math.random() * 100),
d = parseInt(Math.random() * 100);
dataset = [a, b, c, d];
console.log(dataset);
};
// simullate live data input
var update = setInterval(updateData, 1000);
I want to update the chart every 2 seconds.
For that I need a update function that gets the new dataset and then animates a transition to show the new results.
Like that:
var updateVis = function(){
..........
};
var updateLoop = setInterval(drawVis,2000);
I don't want to simply remove the chart and draw again. I want to animate the transition between the new and old bar height for each bar.
Checkout the fiddle

Answer 1

Since your not changing the number of bars, this can be as simple as:
var updateVis = function(){
svg.selectAll(".input")
.data(dataset)
.transition()
.attr("y", function(d) {
return y(d);
})
.attr("height", function(d) {
return h - y(d);
});
};
Updated fiddle.
But your next question becomes, what if I need a different number of bars? This is where you need to handle enter, update, exit a little better. You you can write one function for initial draw or updating.
function drawVis(){
// update selection
var uSel = svg.selectAll(".input")
.data(dataset);
// those exiting
uSel.exit().remove();
// new bars
uSel
.enter()
.append("rect")
.attr("class", "input")
.attr("fill", "rgb(250, 128, 114)");
// update all
uSel
.attr("x", function(d, i) {
return i * (w / dataset.length) + 2.5/100 * w;
})
.attr("width", w / dataset.length - barPadding)
.attr("height", y(0))
.transition().duration(750).ease("linear")
.attr("y", function(d) {
return y(d);
})
.attr("height", function(d) {
return h - y(d);
});
}
New fiddle.

Answer 2

That's the way to go.
Just think what you've done to get the initial chart:
1) Get data
2) Bind it to element (.enter())
3) Set element attributes to be function of the data.
Well, you do this again:
In the function updateData you get a new dataset that's the first step.
Then, rebind it:
d3.selectAll("rect").data(dataset);
And finally update the attributes:
d3.selectAll("rect").attr("y", function(d) {
return y(d);
})
.attr("height", function(d) {
return h - y(d);
});
(Want transitions? Go for it. It is easy to add in your code but you better read this tuto if you want to deeply understand it)
Check it on fiddle