How to create HTML Tooltips using the Charts module's API with Google Charts?
The most recent Charts version does not yet support "tooltips" as such. However, have a look at Pass library-specific options via existing #raw_options, which provides an alternative (using #raw_options) to what you're trying to get to work. For charting engine Highcharts it's already supported (and committed to the most recent dev version), while for charting engine Google Charts you'd need to apply the RTBC-patch attached to comment nr 9 (from about 6 months ago). Obviously, after that patch is committed, my recommendation here to use the #raw_options will continue to work (for either charting engine). And rest assured, this patch will make it (so it's not a patch that might make it).

With this #raw_options solution, we have a solution for using options that are specific for either of the supported charting engines (Google Charts or Highcharts). While still not deviating from a key design concept of the Charts module, which is that all supported charting options should work for either charting engines (you should be able to easily swap from one to the other charting engine, without your charts being affected).

Disclosure: I'm a co-maintainer of the Charts module.