What you are experiencing is an inherent restriction with set analysis. Set analysis is calculated once per chart, and does not understand what dimension it's currently in. You might want to use "total" with a "<>" (subtotal) qualifier, in combination with some conditional function(s), to achieve your results.
Thanks for the responses. Swuehl - I just tweeked it a little bit by including Service into the total <>
The final calculation that works out cumulative frequency to the median used for finding medians for grouped data within a service and month dimension is:-
=count(aggr( if([Wait]< Floor(Median(total<Month,[Service] > [Wait]),1), [ID]), Month,[Id],[Service]))