Easily add vertical line to chart

[GoodMorgon]

Hello,

First off, great package! (long-time fan of dygraphs)

Is there a way to easily add a vertical line to a graph? I've checked the API and docs but there didn't seem to be a documented way, but I'm thinking it shouldn't be that hard...

Thanks!

[GoodMorgon]

Closing for #119

[leeoniya]

you simply draw on self.ctx in a hook of your choice.

i assume you figured it out :)

[leeoniya]

Closing for #119

#220 actually seemed to work correctly when i tested it a few months back. i wanted to first wrap my head around exactly how it worked prior to merging. cc @EmiPhil :D