I listed some remaining problems with this approach here #805 (comment):

I'm realizing now that converting things like the coordinates and sizes of the tooltip in coordinate space necessitates already knowing the axis limits...

I think the best solution is to have the tooltips not be considered at all in the axis calculation, which can be achieved by using double.NaN (although I think changing it so that it would accept null may be more appropriate). This does mean that auto-axising can cut off a tooltip, but this isn't that much of an issue because tooltips automatically flow rightwards if there isn't enough space on the left. One can consider a multi-pass AxisAuto function if this is a concern.