tooltip to support customization of value (rather than just point.y)
If point.y is "1024000", I will like to easily customize the tooltip to show something like: 1024K, 1Mbps, 1.023e6, etc. instead of "1,024,000".
At this moment, I can only create a whole brand new "formatter" function. It would be much better if we can have a value_formatter just to craft the format for point.y.