Allow disabling of tooltips on a per serie basis for LineChart

```
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 `phil.tro...@gmail.com` on 15 Nov 2012 at 9:48
