#Issue 333: Pass point index to drawHighlightPointCallback and drawPointCallback

Currently there is no way to quickly retrieve the index of a point to be drawn/highlighted. 
It is possible to retrieve the x and y values via toDataXCoords() but sometimes it is useful to get the index of the row. 

I have following use-case: 
I have a DataTable consisting of multiple columns. I only want to display one series (columm 0 = X-Axix, colum 1 = y-Axis). The remaining columns have some annotations. 
I want to display different shapes for each point based on the annotation stored in column 3 -N in the DataTable.