Oscilloscope Mode lets you see the sensor values for the sensors connected to the EV3 Brick in real time.
Start/Stop Oscilloscope Mode to turn it on and off.
When Oscilloscope Mode is enabled, the Graph Area will turn blue and any existing graphs will be faded into the background. Data plots will be shown as dotted lines and will update continuously. The following image shows data from two Motor Rotation sensors, for a robot being pushed forward.
Distance Time Graphs Through Data Logging by Erin Land