After a long period of operation or after waking from sleep mode, there can be situations where the history graphs for in- and outgoing data on the network interfaces supposedly doesn't seem to work any longer. No noticeable curves or peaks are visible.
This does not indicate a flaw in System Monitor, but truthfully represents the actual behavior of the computer. The standard setting for the display of network history graphs is Automatic Zoom: Scale to peak since start. In this mode, a maximum peak of the graph represents the highest bandwidth the application was able to monitor during operation. The bottom edge of the graph represents a zero value. Particularly when using wireless networks, radio interferences or errors in the operating system can cause the actual network speed to decrease compared to the initial situation at startup. This is especially true after waking up from sleep mode, when the WiFi interface might automatically connect using a different radio channel, or switches to an alternative frequency range. In that situation, the bandwidth could decrease so far that it becomes almost zero, measured relatively to the situation before. These low values can no longer be presented in the selected scale. They line up on the zero axis and disappear.
If you like to change this behavior, switch the preference for network graphs to Automatic Zoom: Magnify displayed interval. In this case, the graph shows a (perhaps exaggerated) representation of the data transfer history and the scale is being optimized continuously.
Further information on the meaning of the zoom preference setting can be found in the chapter Monitoring Network Interfaces.