What problem does this feature solve?

If user hovers over the series of specific data points. I want the tooltip to display also interpolated values in between of real data points.

What does the proposed API look like?

1. First allow tooltip to be triggered at any mouse position the user hovers.
2. Allow tooltip formatter to contain parameters of interpolated value
3. Allow tooltip formatter to contain parameters to indicates that its an interpolated value
4. Make this feature be activated via flag. Default inactive.