This is my kludgy workaround: instead of
NOW use your data cursor, the z-value is the index of the point you are currently attached to. Voila!
I do wish this was an easier/more elegant change to the DataTip tool, since it would also be useful to use a standard plot command but with a third parameter variable that does not get plotted but does show up in the tooltip; useful for parametric equations. Something like my made-up 'plotParametric':
theta = linspace(0,2*pi,101); R = 1;
[x,y] = pol2cart(theta,R);
where now the tooltip will also display the parameter value theta at a given x-y point. Replace this with index, and it is your above problem.