# How to transition along a path partially or by percentage only

I am a beginner with d3 and currently can not use the newest version, but instead I am on version 3.x.

What I am trying to realize should be simple, but sadly I didnt find resources on how to do it:

The goal is to display a path in my svg. Then I want to display e.g. a circle and transition / move / trace the circle along the path. This works fine if I want the full path to be followed.

But the goal is to follow the path only partially.

What could I do if I want the circle to start from position 0 of the path and follow it until e.g. 25% of the path?

And then again, if the circle is at 25%, how can I follow from there to 50% of the path without starting over from position 0 of the path?

I will be very thankful for any input you can provide here. Thank you very much. I built my answer using this code from Mike Bostock (which uses D3 v3.x, as you want): https://bl.ocks.org/mbostock/1705868

First, I created a data array, specifying how much each circle should travel along the path:

``````var data = [0.9, 1, 0.8, 0.75, 1.2];
``````

The values here are in percentages. So, we have 5 circles: the first one (blue in the demo below) will stop at 90% of the path, the second one (orange) at 100%, the third one (green) at 80%, the fourth one (red) at 75% and the fifth one (coloured purple in the demo) will travel 120% of the path, that is, it will travel all the length of the path and 20% more.

Then, I changed Bostock's function `translateAlong` to get the `datum` of each circle:

``````function translateAlong(d, path) {
var l = path.getTotalLength() * d;
return function(d, i, a) {
return function(t) {
var p = (t * l) < path.getTotalLength() ?
path.getPointAtLength(t * l) : path.getPointAtLength(t * l - path.getTotalLength());
return "translate(" + p.x + "," + p.y + ")";
};
};
}
``````

The important piece here is:

``````var l = path.getTotalLength() * d;
``````

Which will determine the final position of each circle. The ternary operator is important because of our last circle, which will travel more than 100% of the path.

Finally, we have to call the transition, passing the datum and the path itself:

``````circle.transition()
.duration(10000)
.attrTween("transform", function(d) {
return translateAlong(d, path.node())()
});
``````

Here is the demo:

``````var points = [
[240, 100],
[290, 200],
[340, 50],
[390, 150],
[90, 150],
[140, 50],
[190, 200]
];

var svg = d3.select("body").append("svg")
.attr("width", 500)
.attr("height", 300);

var path = svg.append("path")
.data([points])
.attr("d", d3.svg.line()
.tension(0) // Catmull–Rom
.interpolate("cardinal-closed"));

var color = d3.scale.category10();

var data = [0.9, 1, 0.8, 0.75, 1.2];

svg.selectAll(".point")
.data(points)
.enter().append("circle")
.attr("r", 4)
.attr("transform", function(d) {
return "translate(" + d + ")";
});

var circle = svg.selectAll("foo")
.data(data)
.enter()
.append("circle")
.attr("r", 13)
.attr("fill", function(d, i) {
return color(i)
})
.attr("transform", "translate(" + points + ")");

circle.transition()
.duration(10000)
.attrTween("transform", function(d) {
return translateAlong(d, path.node())()
});

// Returns an attrTween for translating along the specified path element.
function translateAlong(d, path) {
var l = path.getTotalLength() * d;
return function(d, i, a) {
return function(t) {
var p = (t * l) < path.getTotalLength() ?
path.getPointAtLength(t * l) : path.getPointAtLength(t * l - path.getTotalLength());
return "translate(" + p.x + "," + p.y + ")";
};
};
}``````
``````path {
fill: none;
stroke: #000;
stroke-width: 3px;
}

circle {
stroke: #fff;
stroke-width: 3px;
}``````
``<script src="//d3js.org/d3.v3.min.js"></script>``