The report would count the maximum number of work items or tasks (depending on the view) in a selected status(es) for a given time. The report would be filterable by one or more statuses and grouped by date (Year, Month, Day) and the WorkItem Type (Story/Defect).

This report would be helpful in evaluating the teams performance when using a Kanban approach to ensure that they are adhering to WIP Limits. The feature currently exists on a board in the UI for the current point in time, but to track history and hopefully see improvement doesn't seem to be available.