v_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_center);
    chebyshev_context.lineTo(w - 12, chebyshev_client_y_center + 5);
    chebyshev_context.moveTo(chebyshev_client_x_center, h);
    chebyshev_context.lineTo(chebyshev_client_x_cente