Plot f(x) or f(y) on screen.

plot.glsl

float plotX(vec2 st, float pct){
  return  smoothstep( pct-0.01, pct, st.y) - 
          smoothstep( pct, pct+0.01, st.y);
}

float plotY(vec2 st, float pct){
    return smoothstep( pct - 0.01, pct, st.x) -
        smoothstep(pct, pct + 0.01, st.x);
}