code: function pointAt( fn, t, pos, size ) {
    var v = (fn == Shaping.step) ? Shaping.step( Shaping.quadraticOut, 6, t, size.y) : fn( t, size.y );
    return new Point( pos.x + t*size.x, pos.y + v )
  }
docs: Use a shaping function (`fn`) to find the value at `t` for this bounding box with (`pos`, `size`)