The Network Monitor utilises a stacked area graph to illustrate the bandwidth usage in your home. 

The area of the colours represented in the graph show the amount of data being used by that device. For example, the yellow spike towards the right of the image isn't using ~120kbit as you can see on the y axis. The peak of the yellow spike ~120kbit minus ~5-10kbps for the blue and purple devices beneath the yellow device. So the yellow device is using ~110kbit.

The graph reads from right to left, the most recent data will appear on the right side of the graph with older data going towards the y axis. The speed of the real time data is dependent on the refresh rate you selected.