Prasad Pandit designed this and shows at his Youtube channel:
I have designed an Oscilloscope using Arduino Mega 2560 and 128×64 GLCD. It works pretty good for verification of signal the range of input frequency is upto 10-12MHz and Sampling rate is 200KSPS. It has no Attenuation so you will have to convert highvoltage signal to 5 v pTp signal. And yes for Sine wave or Cos wave or signal which is below ground voltage you will have to DC shift the signal. You can set the Time/Division for studying signal. It gives Peak to Peak voltage value of signal in Real Time till two decimals.
DIY Oscilloscope using Arduino and Graphic LCD – [Link]