Steps to Reproduce

same example as ZKCHARTS-80

drilldown, drillup, drilldown, drillup repeat()

Current Result

the collection Charts._drilldownLevels increases in size with every drilldown/drillup iteration (consuming memory)

Expected Result

cleanup the drilldown state automatically

Debug Info

Root Cause

the onPlotDrillup event is not fired without a listener leaving the _drilldownLevels not cleaned up

Workaround

adding an empty onPlotDrillup listener fixes the problem