Interpolated fragments in GLSL

Hello,
I'm working on this spectrum visualiser and I want to write it in a shader.
I've gotten stuck with avoiding the interpolation between fragments. I tried getting the floor of a scaled up UV to be my pixels but that didn't work.
To be honest it looks alright with the interpolation but it's not really what I'm going for and I'd like to know how to turn it off.

I've included the jit.gen patch as a reference too.

Thanks!