Today's plugins must use a custom solution to do rate calculation, there's no readily available API for checking against the latest results.
The most common way of doing this, is to store to a file on disk, this is heavily I/O intensive when there's 1000's of checks a minute doing this.
Suggestion: Create an API to graphite, via a few extra paramters, either available externally or handled inside shinken, allow for calculation of rates with treshholds. Example, disk space increased by 5% last 2 hours. Example, network bandwidth increased by 50% since last check.
It should be made more advanced too, ie supporting the UOM metrics (b/B/KB/MB/GB..) and whatnot.