Chris sent in this issue via email:

I think the problem can be re-created pretty easily - generate a plot with two points where the X-values are OA date values, about 10 seconds apart.

Plot.plt.PlotScatter(time1, 10.0);
Plot.plt.PlotScatter(time2, 11.0);
Plot.plt.Ticks(dateTimeX: true);
Plot.plt.AxisAuto();
Plot.Render();

then add a handler for Plot.MouseMove and look at Plot.mouseCoordinates - in my case the values I saw in Plot.mouseCoordinates were all over the place, often outside the bounds of the plot itself.