That sounds like it would be really useful for me too.
Perhaps there’s a way for you to get to this already - what if you ‘tagged’ your charity donations by adding a particular emoji (e.g. 
) to the transaction’s notes. Then if your search for that emoji (& apply your timeframe filter), you’ll see the total & average spent for all of those transactions.
This is exactly the type of thing that Monzo wants to be able to tell you (along with comparing your travel costs / utility bills / groceries etc. to other user’s) 
Hugo’s described what that might look like (& why we don’t have custom categories) here -
https://community.monzo.com/t/custom-categories-multi-category-transactions/649/14?u=alexs