Custom legend for pie charts- size of pies

[springrider]

Good afternoon

I have been evaluating many OS GIS desktops in order to replace our current proprietary software (MapInfo) and QGis seems to be relevant to our needs.
One of our criteria was to be able to load a CSV file and .tab mapinfo files using a project and represents the data from the CSV file as pie charts with the corresponding legend.

Concerning the legend, is it possible to built a legend for proportional symbols or pie charts or diagram overlays? In fact, it continues showing a simple dot.
And extensively, is there any way to customize the legend of a view?

Moreover, concerning the size of the pie, is there only the "linearly scaling" classification or can we add other classification types? In fact some of my pies are really small and it's getting difficult to distinguish them.
Please find attached a  screenshot of my map:
-The legend is not built on the pie charts
-The pies having a small size are almost invisible.

Thanks for your help

Sylvain