Hi Sharon Nathaniel,
With a category axis, I can also replicate the issue you're seeing. As Jan mentioned, it's not really possible to interpolate between categories, as they are discrete values. ShinobiCharts should probably handle this configuration better, maybe by disabling crosshair interpolation if one or both of the axes are category axes.
When I change the x axis to be a numeric axis rather than a category axis, I'm afraid I've been unable to replicate the issue you're seeing. I've taken the code you posted, and plotted some sample data in a chart. When I enable crosshair interpolation, the crosshair behaves as expected.
If you'd like, I could send you the test project I've created, and you could see whether that works for you? If so, it might be worth looking at what is set up differently between the two projects. If you don't feel that the test project covers the use case you're after, maybe you could modify it to demonstrate the problem, and then send it back to us? That would allow us to replicate the issue at this end.
Let me know what you think.