There are many elements which build the engagement data heat maps that are found in the Analytics Center. Most of them relate to how many times an individual viewer has watched a video on a particular day (based on views per Vidyard cookie).
You will sometimes see a dark green bar to the left side of the graphs. There doesn't appear to be any information in the key about this data.
What you can see is actually a scrubber bar. These dark green lines will only appear when a single video is filtered (or single player containing one video). If you press play on the preview video, the green bar will move along the graph so you can pinpoint exactly when a viewer decided to stop watching.