Now upload this code into your Arduino
After Run this code in Processing IDE
And then you just need to connect the Arduino analog pin 0 to the signal you want to read.
And It's done!
Read more: http://www.instructables.com/id/Arduino-Oscilloscope...
Another one via Open-electronics.org
We have designed an Oscilloscope using PC and Arduino Board. The signal is first of all fed to the Arduino Board where the analog signal is converted to a digital signal by the ADC which is then serially outputted to the PC and is read by the MATLAB software via the COM ports. Here the signal is read in the form of digital data but then is converted to analog one by using the resolution of the ADC used by the Arduino Board. The MATLAB software was then used to plot the signals...
Full project in PDF: DESIGNING A PC OSCILLOSCOPE
Read more: http://www.open-electronics.org/guest_projects/a-pc-and-an-arduino-heres-your-diy-oscilloscope