a common use case for pie graphs would be to use total diskspace for pie graph "max" value and used diskspace as an item on top of that.
obviously, on most systems it doesn't make much sense to monitor total diskspace too often, so once every 12 hours or so might be considered sufficient.
unfortunately, if looking at a pie graph for one hour, only values in the last hour are considered.

to make pie graphs more useful in real world, they could use last collected value (either only for "max" items, or configurable)