Download oscilloscope.ino into the Arduino board. It works well on Arduino Uno.
The inputs are from A0 to A5 (0-5V).
You need python2, python2-pygame and python2-pyserial. If you are on Windows, replace /dev/ttyACM0 by COM1 (or COM2, COM3...).
Run oscilloscope.py (option -f for fullscreen).
Each line represents an input. Enable or disable inputs with keyboard (keys from 0 to 9, but the Arduino can handle only 6 channels for the moment). Pause with space. Get more numbers by moving the mouse cursor.
Accuracy is very limited by the duration of Arduino analogRead function (100 µs), so the more active inputs you have, the slower it is.