Workaround for tooltips remaining enabled on points filtered out

This is one approach to fixing #246 (closed) although I'm not sure it's the best one; as mentioned in the commit this change only avoids displaying the tooltip once the mouseover has been triggered.

I think a cleaner, and probably more performant, way to fix this would be to disable those points properly (so the mouseover events aren't even triggered). However I wasn't sure which way was preferred and don't know the model well enough to make that change immediately. This gets the conversation started at least!