Currently there is no way to select the date range in the 'server calls usage' interface. I cannot look back at my historical data. The simple solution is to make the date range something the user can select rather than a fixed range.
So if you click download from the Report Suite tab you can select a single date range. But it would be more useful to me to be able to trend the server calls over a time range (e.g. monthly over last 12 months). Similarly, adding a third line to the graph that trends your allotted call limit (instead of just current year vs prior) would be helpful. But ultimately for our business the running total isn't especially useful and I would rather graph daily or monthly usage.
