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.
All the Arduino GLCD libraries used are freely available. If you have suggestions mail me on: email@example.com
Code Link : https://db.tt/itk4CLwi
DIY Oscilloscope using Arduino and Graphic LCD – [Link]