Comment by pjtr

Same with leaning. Add one more line of code and tweak the draw call:

    var ylean = (input.leftright*(i/screenwidth-0.5) + 0.5) * screendata.canvas.height / 4;
    DrawVerticalLine(i, heightonscreen+ylean, hiddeny[i]+ylean, map.color[mapoffset]);

It adds a lot to the "feeling" IMO :)