Cool. One issue: not all wave shapes play at the same volume.
However, sawtooth and square are still louder because they're made of three osillators (spread based on touch surface).
Another reason for the difference between waveforms is multitouch, this is also something that sounds different for different waveforms.
Waveforms having the same amplitude doesn't mean the ear hears it at the same volume. There's more frequencies contained in different waveforms and the ear is more sensitive to certain ranges.
A smoother representation of the waveforms might contain the first 5 - 10 frequencies of Fourier series but even then you need to compensate with a change in amplitude.
It's not a DAW, it's a web app, and the users will appreciate the favor.