# D3 put arc labels in a Pie Chart if there is enough space

I will put a text element in every arc of my Pie Chart (center) - as shown in this example: http://bl.ocks.org/mbostock/3887235

But I will only put the text element if the room is sufficient for the whole text, so im must compare the size of my text element with the "available" space in every arc.

I think I can do this with getBBox() to get the text dimensions... but how can I get (and compare) the dimension of the available space in every arc.

thx...! This question has been asked several times before.

The solutions I have suggested there 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.

One of the other solution, the one suggested by Lars, is to put the labels outside the pie chart. However, that just pushes the labels outside, granting them a larger radius, but does not solve the `overlap` problem completely.

The other solution is actually using the technique you suggest: just remove the labels which do not fit.

# Hide overflowing labels

Compare Original, which has `>= 65` label overflowing to Solution where the overflowing label is gone.

### Reducing the problem

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).

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. 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
theta1 = d3Arc.startAngle()(ptData),
theta2 = d3Arc.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);
}
``````

## 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:

``````g.append("text")
.attr("transform", function(d) { return "translate(" + arc.centroid(d) + ")"; })
.attr("dy", ".35em")
.style("text-anchor", "middle")
.text(function(d) { return d.data.age; })
.each(function (d) {
var bb = this.getBBox(),
center = arc.centroid(d);

var topLeft = {
x : center + bb.x,
y : center + 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. We first 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. Once we have the center, we just calculate the `topLeft`, `topRight`, `bottomLeft` and `bottomRight` points from it and see whether they all lie inside the `wedge`.

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

Original

Solution

## Note

1. I am using the `innerRadius` which, if non zero, makes the `wedge` non-convex which will make the calculations much more complex! However, I think the danger here is not significant since the only case it might fail is this, and, frankly, I don't think it'll happen often (I had trouble finding this counter example): 2. `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 `Math.atan2` `? = Math.atan2(y, x) = Math.atan2(-svg.y, x)`

Coordinate system for `d3.svg.arc` `? = Math.atan2(x, y) = Math.atan2(x, -svg.y)` You can't really do this with the bounding box because the bounding box is much larger than a wedge for the pie chart wedges. That is, even though the wedge at the outer edge would be wide enough to accommodate the text, that doesn't mean that it's wide enough at the actual position of the text.

Unfortunately, there's no easy way of doing what you're trying to do (pixel-level overlap testing). See e.g. this question for some more information. I would suggest simply putting the text labels outside of the pie chart so you don't run into this problem.