You might want to consider setting up summary indexes - for example, based on a daily search, then you can base your dashboard on searches of this index rather than having to keep all your raw data. Depending on space availability, you could keep the summary indexes for a lot longer than the raw data but still get useful information out of them such as comparing your current daily counts against daily counts from last week/month/year etc. Obviously, you need to consider which information to keep in your summary index, but it is likely to be a lot smaller than your raw data.
