DSPTutorial - Why is Oscilloscope clipping?

In the Introduction to DSP tutorial, there is besides the on-screen keyboard, an oscilloscope and a spectrum analyzer. This are both cool and something I want to implement in my first synthesizer plugin.

However when I strike a key on my MIDI keyboard real fast (velocity), the scope is clipping the sine wave. That does not seem right to me. The scaling seems off. Is there any way I can easily adjust the scale so it does not clip?