It is a good idea and it was requested before in different forms: Specialised allocations. Including sectors, diversification & Portafolio grouped by sector
Also combined with what @Vedran was proposing in Portfolio graph, probably a nice grouping feature can be implemented at the Allocation section from Portfolio tab.
I think it can copy the customization idea of My Watchlist from main tab and also allow different view graphs (except circle/“pie” one), as example: stacked area chart.
Hopefully we will see something along those lines released soon. 