@chriddyp Thanks for coming back to this issue after such a long time. I wrote my comment under the github discussion.
@jimmybow
Is there any way to get the latitude/longitude of the mouse hover point, rather than just the ‘x’ and ‘y’ values?
Hi, you can use visdcc.Run_js to write your own javascript event listener. 
here is the example code https://github.com/jimmybow/visdcc/blob/master/example/Run_js/Add_event_and_callback.py
Hi, this is a good example of mouse hover but is it possible to update the string store location and year to the map hover itself instead of returning it to the html div element.