The 6D Ackley function has many local minima and maxima and this is reflected in the complexity of the tree. The graph layout algorithm tries to lay out the tree so that the y-position of the nodes relates to the function value. The contour tree only shows extrema and saddle points so these are the only points possible for which we can find the function value. In addition, since the points have no particular organization with respect to their input values one must scan through all the points to find a particular input value configuration.