Move widget along a line

Hi there

I’d like to demo an interactive graph, where a vertical line moves across it on hover to show a specific value. I’ve got this working, but I’d like the dot to stick to the turquoise line and follow it as the mouse moves, whilst the straight blue line just moves horizontally (Image and file attached explain it a bit better).
Graph Hover.rp (49.9 KB)

Any ideas?