Can't make paths draw growing slowly with D3

Question

Using the d3 graphics library, I can\'t seem to make paths draw slowly so they can be seen growing.

This site has a perfect example in the \"Line Chart (Unrolling)

Answer 1

I believe the "D3 way" to do this is with a custom tween function. You can see a working implementation here: http://jsfiddle.net/nrabinowitz/XytnD/

This assumes that you have a generator called line set up with d3.svg.line to calculate the path:

// add element and transition in
var path = svg.append('path')
    .attr('class', 'line')
    .attr('d', line(data[0]))
  .transition()
    .duration(1000)
    .attrTween('d', pathTween);

function pathTween() {
    var interpolate = d3.scale.quantile()
            .domain([0,1])
            .range(d3.range(1, data.length + 1));
    return function(t) {
        return line(data.slice(0, interpolate(t)));
    };
}​

The pathTween function here returns an interpolator that takes a given slice of the line, defined by how far we are through the transition, and updates the path accordingly.

It's worth noting, though, that I suspect you'd get better performance and a smoother animation by taking the easy route: put a white rectangle (if your background is simple) or a clipPath (if your background is complex) over the line, and transition it over to the right to reveal the line underneath.