Describe the bug

When hovering over a Line/Point in the plot we usually see the tooltip with the x and y values but if the line is near to the top of the plotting area the tooltip is not completely shown anymore:

To Reproduce
Steps to reproduce the behavior:

1. Go to https://emilk.github.io/egui_plot
2. Move the plot in the plotting area so its near to the top and
3. Hover over the line

Expected behavior
The position of the tooltip should switch to the bottom of the mouse so the tooltip can be shown correctly

Screenshots