Cumulative Flow Diagram (CFD) should be able to work in hours as well as number of issues

At present, as far as I can tell, the y-axis of the CFD shows just number of issues. At least for me, NUMBER of issues is really not a reasonable measure of AMOUNT of work in progress. Ideally all issues would be roughly similar in size, but in practice that is rarely the case. Much more useful would be an axis that showed a variable metric, such as hours or story points.