highlight selection in timeline view

[anoopsarkar]

We have always had this issue but we can probably do something about it now.

When we add a constraint (e.g. add "person: Charles Martel") that the lower timeline view (which is always zoomed out to include the full timeline) has the events highlighted but due to the typically smaller number of events the location of those events in the timeline is not apparent to the user.

We can add a faint line to the background of the lower timeline plot that extends from the bottom to the top in order to highlight the events. I hope that even single years spread out over the entire timeline will have increased visibility. I have made a crude illustration below:

This is similar to search in a page in most modern web browsers and other cases where hits are highlighted to enable the user to see at a glance what the query has picked out in the entire set.

[anoopsarkar]

@Vlad003 can you take this on after you are done with integrating the ngram view?