2 separate users have requested this type of hover mode in the Dash community forum. So, I thought I'd create an issue to collect further interest.

https://community.plot.ly/t/vertical-line-on-hover/5321/3

https://community.plot.ly/t/capturing-mouse-events-position/4616/8

Perhaps could be solved as part of #1847 (customized hover shapes) or just as a separate hovermode.