Hi Shaik, thanks for coming back to me.
That doesn't seem to be working for me - the graph shows 100% for all months. Also, the set analysis doesn't look like it would include cases where there is no closure date (i.e. the case is still open)
I've since I last posted,I've also tried making sure there is a GenericMonthEnd field in my autogenerated calendar field to replace the monthend calculation above. This hasn't made life any faster!