D3 transition between line interpolations - d3.js

Question

I have been searching for quite a while but I haven't been able to figure it out.
I want to have a transition between two interpolations "basis" and "step-after" for a single line.
var time_scale, kpi_scale;
var line;
var data = [
{"kpi": 100, "time": 1317448800000},
{"kpi": 200, "time": 1320127200000},
{"kpi": 250, "time": 1322719200000},
{"kpi": 180, "time": 1325397600000},
{"kpi": 230, "time": 1328076000000},
{"kpi": 360, "time": 1330581600000},
{"kpi": 140, "time": 1333260000000},
{"kpi": 120, "time": 1335852000000},
{"kpi": 240, "time": 1338530400000},
{"kpi": 190, "time": 1341122400000},
{"kpi": 185, "time": 1343800800000},
{"kpi": 130, "time": 1346479200000},
{"kpi": 340, "time": 1349071200000},
{"kpi": 320, "time": 1351749600000},
{"kpi": 250, "time": 1354341600000}
];
// set up the viewport, the scales, and axis generators
var container_dimensions = {width: 900, height: 400},
margins = {top: 10, right: 20, bottom: 30, left: 60},
chart_dimensions = {
width: container_dimensions.width - margins.left - margins.right,
height: container_dimensions.height - margins.top - margins.bottom
};
var time_extent = d3.extent(
data,
function(d){return d.time;}
);
time_scale = d3.time.scale()
.domain(time_extent)
.range([0, chart_dimensions.width]);
var kpi_extent = d3.extent(
data,
function(d){return d.kpi;}
);
kpi_scale = d3.scale.linear()
.domain(kpi_extent)
.range([chart_dimensions.height, 0]);
var container = d3.select('body')
.append('svg')
.attr("width", container_dimensions.width)
.attr("height", container_dimensions.height)
.append("g")
.attr("transform", "translate(" + margins.left + "," + margins.top + ")")
.attr("id","chart");
draw_timeseries (data);
transitionToStepped (data);
function draw_timeseries (data) {
line = d3.svg.line()
.x(function(d){return time_scale(d.time);})
.y(function(d){return kpi_scale(d.kpi);})
.interpolate("basis");
d3.select('#chart').append('path')
.attr('d', line(data))
.attr('id', 'line')
.attr('class', 'line');
}
function transitionToStepped (data) {
line.interpolate("step-after");
d3.select("#chart").selectAll(".line")
.transition()
.duration(2000)
.attr("d", line(data))
.delay(2000);
}
I have saved a small jsfiddle containing my code.
jsFiddle Interpolation Transition
I wanted to "morph" between the basis line and the step-after line. But d3 is sliding in the step-after line from the left instead of "bending" the basis line into a step-after one.
Thanks for your help.
Christoph

Answer 1

There has got to be a way to do this, and certainly one better than the offering below (I am looking forward to seeing other answers), but I have seen no answers here as of yet, and this will at least do a "morph" for you, although it also makes the line parallel to the x-axis during the transition. Not a terrible thing, but I believe not precisely what you are after.
Here's a fiddle for the full code and demo,
modified from your original:
http://jsfiddle.net/n5P6z/2/
The meat of the approach is here:
function xLineReduce() {
line.x(function (d) {
return time_scale(d.time);
}).y(function (d) {
return chart_dimensions.height;
});
}
function resetInterpolator() {
line.interpolate("cubic-in-out");
}
function stepItUp() {
line.x(function (d, i) {
return time_scale(d.time);
})
.y(function (d, i) {
return kpi_scale(d.kpi);
})
.interpolate("step-after");
}
function transitionToStepped(myData) {
path.data(myData)
.call(xLineReduce)
.transition()
.delay(1000)
.duration(500)
.attr('d', line(myData));
path.data(myData)
.call(resetInterpolator)
.transition()
.delay(1500)
.duration(0)
.attr('d', line(myData));
path.data(myData)
.call(stepItUp)
.transition()
.delay(1500)
.duration(500)
.attr('d', line(myData));
}