Ability to add custom vertical lines

[marcbowes]

I have a use-case where I'd like to be able to show a particular event happening in timeseries data (using a line chart). For example, consider a line chart that has x=[Monday, ... Sunday] and y=[0, 0, 0, .., 1, 1]. I'd like to show on the graph "start of weekend" around x=Saturday as an explanation for why the y values changed.

I couldn't find a way to do this with the library as is, apologies if I missed something!