Treemap padding introduces bias, penalizing small nodes.

[mbostock]

The treemap implementation subtracts a fixed amount of padding from the computed node sizes. This is simple to implement but disproportionately affects the size of small nodes, since the relative area of the subtracted padding is much higher with a small node than a big node.

In d3-shape’s pie generator, we go to some effort to ensure that the relative areas of padded arcs are preserved. We don’t do that with the treemap layout, in part because I presume it is more challenging to implement, but it would be lovely if we could. A two-pass approach might be an acceptable if imperfect solution: compute the layout without padding, compute the area lost to padding for each cell, add this to the value for the cell, and then recompute the layout.

Perhaps I should start by visualizing the distortion using the Flare hierarchy as an example dataset.

Related d3/d3-shape#41.

[mbostock]

This is an interesting idea but I’m not going to pursue it immediately.