Could you be more specific? Do you have an example of a graph with this feature?

Something like this I guess: http://dygraphs.com/gallery/#g/highlighted-series

👍 that can be very useful, in fact we've implemented this exact feature for clients by using hover/unhover events.

Perhaps this even deserves a button in the mode bar? It would be nice to give viewers the option to turn it on and off while they're looking at the plot - for instance, I can imagine wanting it on while I'm getting an overview of all the traces, then wanting it off while I'm drawing a box to zoom in, or while focusing on two traces simultaneously to study their correlation.

👍 If it is not easy to be added to mode bar, can we make it default on when plotting?

Old issue. Merging with #1847