Display x-coordinate of the intersection of the slice line with the bottom horizontal axis.

[mlesnick]

It was suggested to me that the RIVET GUI display the x-coordinate of the intersection of the slice line with the bottom horizontal axis. And similarly for the y-axis. This could be shown continuously, or using a tool tip.

The need for this would be mitigated at least somewhat by the addition of scale marker on the axis, as in one of the open pull requests. Nevertheless, I think it could be a nice feature.