Allow disabling of tooltips on a per serie basis for LineChart

[GoogleCodeExporter]

I am proposing you add a feature that allows to disable tooltips for only for 
given data series on a LineChart (or Line Chart within ComboChart).

This would allow specific series to display tooltips on their data points but 
other series not to display tooltips at all.

Thanks.

Original issue reported on code.google.com by [email protected] on 15 Nov 2012 at 9:48