What do the two different colors mean in the Data Usage graph?

Question

The Google Dialer app consumed all my data in the background for reasons unbeknownst to me on my Nexus 5. I restarted my phone in safe mode. When I went to check the app data usage again (just to make sure I wasn't crazy), I saw the following:

Notice the two different colors: one a light green, the other a dark green. Why are there two different colors, and what do the colors mean?

The data usage graphs for other apps also have these different colors, so it's not just this graph that's showing like this. I assume there must be some meaning to the shading.

Answer 1

The dark green is the data used by the specific app you are viewing (in this case, the Dialer). The light green is total data usage within the period, by all apps.

The idea is that it allows you to visualize how much data a given app has used relative to your total usage.