It would be nice to have an option in plotly.js to toggle on a vertical or/and horizontal line across the entire plot area on hover. The behavior of the line should be similar to the behavior of the spikes feature, but there are several differences: * The line shouldn't depend on hovermode (as spikes do at the moment), moreover it can be used in 'x' or 'y' hovermode to compare data on hover. * This line should be always drawn over the nearest x/y point, regardless of whether this point is within the max hover distance or not. This feature could be very useful for financial and stock charts Related to #2026, #1959 PR #2150