Friday, November 8, 2013

Hide overflowing labels on pie charts

I have seen many questions related to occlusion of labels on pie charts and have always suggested that the problem should be solved by moving the labels outside the pie chart, or by rotating them to align with the axis.

The solutions I have suggested often is to rotate the label but it has never quite satisfied me. Part of it was the horrible font rendering done by some browsers and loss in legibility that brings and the weird flip when one label crosses over the 180° line. In some cases, the results were acceptable and unavoidable, e.g. when the labels were too long but in most cases, they looked just clumsy.

I would consider using a different form of visualization for the data usually, but out of curiosity, I thought that I should try to see how difficult is it to actually do it.

The results are on this Plunker.

Hiding overflowing labels

Reducing the problem

Iff all corners are inside then no overlap
between the convex polygons 
The key insight is to see that this problem is of finding whether one convex polygon (a rectangle, the bounding box) is contained inside another convex polygon(-ish) (a wedge of a pie chart).

The problem can be reduced to finding whether all the points of the rectangle lie inside the wedge or not. If they do, then the rectangle lies inside the arc.

Does a point lie inside a wedge

Now that part is easy to do.

All one needs to do is to check:

  1. The distance of the point from the center is less than the radius
  2. The angle subtended by the point on the center is between the startAngle and endAngle of the arc.

function pointIsInArc(pt, ptData, d3Arc) {
  // Center of the arc is assumed to be 0,0
  // (pt.x, pt.y) are assumed to be relative to the center
  var r1 = arc.innerRadius()(ptData), // Note: Using the innerRadius
      r2 = arc.outerRadius()(ptData),
      theta1 = arc.startAngle()(ptData),
      theta2 = arc.endAngle()(ptData);
  var dist = pt.x * pt.x + pt.y * pt.y,
      angle = Math.atan2(pt.x, -pt.y); // Note: different coordinate system.
  angle = (angle < 0) ? (angle + Math.PI * 2) : angle;
  return (r1 * r1 <= dist) && (dist <= r2 * r2) && 
         (theta1 <= angle) && (angle <= theta2; }
innerRadius > 0

I have included one more check here: whether the point likes outside the innerRadius of the arc. This causes some complications I have discussed below.

Find the bounding box of the labels

Now that we have that out of the way, the second part is figuring out what are the four corners of the rectangle. That, also, is easy. SVG provides us with the convenient getBBox(). To do exactly that.

    .attr("transform", function(d) { return "translate(" + arc.centroid(d) + ")"; })
    .attr("dy", ".35em")
    .style("text-anchor", "middle")
    .text(function(d) { return; })
    .each(function (d) {
       var bb = this.getBBox(),
           center = arc.centroid(d);
       var topLeft = {
         x : center[0] + bb.x,
         y : center[1] + bb.y
       var topRight = {
         x : topLeft.x + bb.width,
         y : topLeft.y
       var bottomLeft = {
         x : topLeft.x,
         y : topLeft.y + bb.height
       var bottomRight = {
         x : topLeft.x + bb.width,
         y : topLeft.y + bb.height
       d.visible = pointIsInArc(topLeft, d, arc) &&
                   pointIsInArc(topRight, d, arc) &&
                   pointIsInArc(bottomLeft, d, arc) &&
                   pointIsInArc(bottomRight, d, arc);
    .style('display', function (d) { return d.visible ? null : "none"; });
The pith of the solution is in the each function. First, we place the text at the right place so that the DOM renders it. Then we use the getBBox() method to get the bounding box of the text in the user space. A new user space is created by any element which has a transform attribute set on it. That element, in our case, is the text box itself. So the bounding box returned is relative to the center of the text, as we have set the text-anchor to be middle.

The position of the text relative to the arc can be calculated since we have applied the transformation 'translate(' + arc.centroid(d) + ')' to it. Then we just calculate the topLefttopRightbottomLeft and bottomRight points from it and see whether they all lie inside.

Finally, we determine if all the points lie inside the wedge and if they do not fit, set the display CSS property to none.

This was fun.


I am using the innerRadius which, if non zero, makes the wedge non-convex. Now the correct calculations will be much more complex!

The failure case
Labels may overflow when innerRadius > 0
However, I think the danger here is not significant since the only case it might fail when the inner curve impinges into the first character, and, frankly, I don't think it'll happen often.

Secondly, x and y are flipped and y has a negative sign while calculating Math.atan2. This is because of the difference between how Math.atan2 and d3.svg.arc view the coordinate system and the direction of positive y with svg.

Coordinate system for <code>Math.atan2</code>
Coordinate system for Math.atan2
θ = Math.atan2(y, x) = Math.atan2(-svg.y, x)

Coordinate system for <code>d3.svg.arc</code>
Coordinate system for d3.svg.arc
θ = Math.atan2(x, y) = Math.atan2(x, -svg.y)

Working demo


The stackoverflow question which prompted this post was this.


Anonymous said...

You blatantly plagiarized this from stackoverflow. What the heck, man?

Anonymous said...

You blatantly plagiarized this from stackoverflow. What the heck, man?

Utkarsh Upadhyay said...

I am the one who answered the question on SO and I have added a note on the bottom which says so. :)