Regarding the histogram issue, I worked on a project that had a few hundred histograms based on data from over 3 billion data points. It turns out that after a few thousand data points many histograms will stop changing significantly.

So, unless you really need to show exactly how many data points each bucket contains, it's much easier to run the analysis once offline, then serve just the histogram percentage data. From that you can make an SVG and overlay additional user-specific data on top. The point is that this histogram data is small and easy to cache.

You can then rerun the histogram analysis later if you'd like. However, for this project I never saw anything change with more data. It was overkill even to run it as a cron job.