Actually, my confusion was not due to the Y axis being labeled "G" (for giga) instead of "B" (for billion) - that actually makes sense - "G" is widely recognized to mean billion.
The problem with the graph is that the Y axis units are in bytes (actually, billions of bytes) rather than in bytes/second (or some other time unit). To understand the issue, look at the graph at time 15:15 - the orange line indicates a y-axis value of approximately 8.2 Gigabytes at 15:15. But, that doesn't really make sense - 15:15 is an instant in time - clearly 8.2 gigabytes were not transferred at exactly 15:15. So, what is indicated? 8.2 gigabytes/second would make sense, as would 8.2 gigabytes/minute (both of these are rates of data transfer). But, 8.2 gigabytes at 15:15 does not really make sense.