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){
        A[i] = [];
        for(var j = 0; j < runge_n; ++j){
            A[i][j] = Math.pow(runge_xs[i], j);
        }
        B[i] = runge_ys[i];
    }

    X = solved_linear(A, B, runge_n);

    