Summing issue level custom measures

Hi. I have created a custom measure to convert an issue-level, pick list, custom field into an integer value. It works via a case statement. It’s a risk severity value. I’d like to create a week by week bar chart of the sum of this value for all issues. However, because my measure is only working at issue level I get an empty chart when I simply use time as the row and my measure as the column. Is it possible to populate the hierarchy with appropriately summed values?