[partition] Improve treemap style

[markov00]

We should consider a redesign of our treemap to align them a bit more to the style used in the metric chart.
This is for example our current design

And this is where we should probably go

[gvnmagni]

To be considered: #2306

[markov00]

It will be interesting to consider how are we going to handle the rectangle resizing: what do we consider most important: the title or the value, do we need to reduce the size of both, do we need to rescale the value, truncate the text?