This feature is very important. Currently it is easy to get data for a given day for a given report.
We need to be able to easily access data for a set of dates, eg - the last 30 days - the last month - the last week - week 2 - month of march 2007
This has to be implemented in such a way that this is globally available to all APIs and plugins without hack. This is part of the core.
Once this feature is ready we can imagine to easily generate RSS feeds for everything, giving reports for the last N days (see #5459).
done in  see http://piwik.org/demo/?module=API&action=listAllAPI