The expression for the mini chart will be the expression which you write.
Say for example you have written expression as Sum(Sales) and you select the representation as Mini chart then for mini chart the expression will be sum(Sales).
Hope this is clear.
Regards,
Kaushik Solanki
Please remember to hit the 'Like' button and for helpful answers and resolutions, click on the 'Accept As Solution' button. Cheers!