runge_n; ++i){
        runge_context.moveTo(x_to_client(runge_xs[i])-5.5, y_to_client(runge_ys[i])+5.5);
        runge_context.lineTo(x_to_client(runge_xs[i])+5.5, y_to_client(runge_ys[i])-5.5);
        runge_context.moveTo(x_to_client(runge_xs[i])+5.5, y_to_client(runge_ys[i])+5.5);
        runge_context.lineTo(x_to_client(runge_xs[i])-5.5, y_to_client(runge_ys[i])-5.5);
    }
    runge_context.strokeStyle="#AA3311";
    runge_context.stroke();
    runge_context.closePath();

    // guides
    if(do_guides