ntListener('mouseleave', function(e){
        runge_draw(0, 0, false);
    }, false);
}

function runge_draw(client_x, client_y, do_guides){
    runge = document.getElementById("runge");
    var runge_context = runge.getContext("2d");
    runge_context.font = "16px sans-serif";
    // background
    runge_context.fillStyle="#eeeeee";
    runge_context.fillRect(0, 0, w, h);

    // grid
    draw_grid_on(runge_context);

    // runge
    var A = [];
    var B = [];
    for(var i = 0; i < runge_n; ++i){
      