For this last graph Webserver Stress Tool constantly measures vital parameters of the machine it runs on. It can be helpful to find out if the limits of the test client have been reached.
Especially the line for the CPU Load (pink) should be well below 100%. If you constantly hit values above 90% for the CPU load the test results may be incorrect.
Also the network traffic (blue line) should be below the physical limits of your connection to the server.
