Why do you think that is a lot? Most graphs (think about the daily/weekly graphs) are based on RRD, which means "Round-Robin-Database". Its a Database of a fixed size, for each new entry the last one gets deleted. Therefore its normal that there is a significant use of space even after just one day of operation after these databases have been initalized.
But please re-check after some days just to make sure there is no space-hog.
