I ran into this problem today. One workaround, as suggested by Kun02, is to implement your own "Clear Chart" button and when that button changes to True, write an empty array to the chart history via a property node. Then the memory usage drops each time the "Clear Chart" button is pressed.
Doesn't keep the user from right clicking on the chart, selecting Clear Chart from the built in popup menu and using more memory though.
Seems like this should be an easy fix for NI to incorporate into LabVIEW.