PlotLine event listener receives an event target which is a Charts instead of PlotLine

Steps to Reproduce

1. run the attached zul
2. hover on the red, horizontal plotline

Current Result

the PlotLine event listener receives a MouseEvent, call event.getTarget(). It returns a Charts object

Expected Result

The PlotLine objects that listens to the event

Debug Information

• a use case like ZKCHART-52, needs to manipulate a plotline upon its event.