  chebyshev_draw(0, 0, false);

    chebyshev.addEventListener('mousemove', function(e){
        var canvas_rect = chebyshev.getBoundingClientRect();
           chebyshev_draw(e.clientX - canvas_rect.left, e.clientY - canvas_rect.top, true);
           if(e.buttons == 1){
            chebyshev_ys[chebyshev_i] = chebyshev_client_to_y(e.clientY - canvas_rect.top);
            chebyshev_draw(e.clientX - canvas_rect.left, e.clientY - canvas_rect.top, false); // double work
           }
    }, false);

    cheby