Request ability to tag events when log graphing

During log analysis I would like to place a labeled marker on graph timeline to designate events that I identify. This is helpful when i later add other elements to the graph that change the scale making the event difficult to recognize. Labels would also be helpful when grabbing screen shots to post for the community.

No files, tests, or entry points are named. Start by locating Mission Planner's log-graphing UI and determine how timeline scaling and rendering work; done means users can place labeled event markers that remain identifiable after adding graph elements and appear in screenshots.

Written by the indexing model from the issue text.