"chebyshev");
    var chebyshev_context = chebyshev.getContext("2d");
    chebyshev_context.font = "16px sans-serif";
    // background
    chebyshev_context.fillStyle="#eeeeee";
    chebyshev_context.fillRect(0, 0, w, h);

    // grid
     chebyshev_context.beginPath();
    chebyshev_context.moveTo(0, chebyshev_client_y_center);
    chebyshev_context.lineTo(w, chebyshev_client_y_center);
    chebyshev_context.lineTo(w - 12, chebyshev_client_y_center - 5);
    chebyshev_context.moveTo(w, chebyshev_client_y_