MRTG graph shape question -- triangle vs rectangle -- with proposed answer

Below is some Multi Router Traffic Grapher (MRTG) output as shown by the SoYouStart Control Panel. What was happening to produce the MRTG graph was that I logged in to an idling server via ssh and ran

# apt-get update && apt-get dist-upgrade -y

My question is: Why is the graph triangular instead of rectangular?

I assume that almost all of the bandwidth represented by the graph was used for downloading of the updates and upgrades and very little represented terminal output via ssh. I assume that the constraint on getting the updates and the upgrades would be the slower of the connection speed from the update and upgrade servers to my server or the upload bandwidth of the update and upgrade servers. It would seem like the process would rapidly reach the constraint and pretty much maintain that speed. The graph seems like it should be a rectangle or at least a rectangle-ish trapezoid. I can imagine the bandwidth varying over time, or varying even within a couple of minutes, but -- like a triangle -- why?

I googled around, but didn't immediately find much. Then I moved my mouse around on the graph and got an idea of what the answer might be.

Proposed answer: The SoYouStart Control Panel seems to collect data points every five minutes. In this case the update process took less than ten minutes. The first and the last data points were near zero because the first happened before the update process started and the last happened after the process ended. The middle data point was somewhere during the process. The graph is triangular because the only real data involved is the three points, which make a triangle. The triangular shape has almost nothing to do with the shape of the bandwidth utilization throughout the update process. To look at the shape of the traffic meaningfully, I would need additional data points.

I've seen these triangular graphs a lot. It's taken me awhile to maybe figure out why.

Thanks everybody! Hope everybody is enjoying the weekend!