How good is the analogy here? The point that Diotima wants to make is: If something's not beautiful, it doesn't follow that it's ugly; if something's not good, it doesn't follow it's bad. But here of course the question arises: if something is neither beautiful nor ugly, then what is it?
Once more, beautiful and ugly are not just simply extreme ends on a simple, one-dimensional scale of some quantitative measure. There is no such thing as 'beauty points', it's not that, when you look at something and assign, say, a hundred beauty points to it, then it earns the title 'beautiful', whereas when you assign a hundred negative points it's then called 'ugly'. It doesn't work that way. Look at these two landscapes:
(If you don't agree with my feeling that the area pictured in the first photo is beautiful and the one in the second is ugly, then insert your own favorite examples.)
Beauty is a fragile thing: we can easily imagine the beauty of the landscape above destroyed by a single element that doesn't fit (for example, if we inserted an oil rig); on the other hand, a single small thing (such as a lone flower or a sudden burst of sunbeams after a rain) can give the ugliest setup an unexpected atmosphere that utterly transforms it from deep ugliness to beauty. (Our perception of people as ugly or beautiful can undergo similarly abrupt and total shifts into the complete opposite.)
Moreover, something (or someone) can be beautiful or ugly in many different ways; what might count as a 'beauty point' in one constellation wouldn't in another one. Have you noticed that in the second picture many of the trees in the background are in full bloom (visible mostly as white patches)? This would bring a certain attractiveness to many other sceneries, but in this case, if they'd been simply lavishly green the view would probably have been less ugly. Beauty and ugliness are constellations in which many elements must fit together in certain ways, and what makes them into beauty or ugliness is neither certain particular elements or a certain quantity of them alone; what makes them into instances of beauty or ugliness is as much in the constellation as it is in the particulars that go into it (fittingness, purity, integrity, and other attributes may play a role here that apply more to the whole than to its parts). Beauty and ugliness are more akin to states of perfection.
Finally, it's not an accident that Plato connects the beauty/ugliness pair of terms with the ethical good/bad contrast and then with an example from the sphere of truth and belief (with its knowledge/ignorance distinction). There is an intimate relationship between what's valuable in all these areas (truth, beauty and goodness). In Plato's world, these ideas cannot be separated.
Just as in correct opinion there is some connection with the truth (after all, correct opinion does hit the truth, it's just that there are no reasons behind it), in eros there is some connection with beauty. It's not, however, that eros is beautiful, but that what eros aims at is beautiful.
Unfortunately, the form of diagrams I have used in my previous posts seems to fail for this case: it's hard to supplement the contrast between beauty and ugliness with some additional dimension, in the same way we supplemented wisdom/ignorance and knowledge/ignorance with the desire for wisdom and the ability to give reasons in order to give more nuanced accounts.
 Do you know the phrase 'things are not as black and white'? That phrase is intended to make the same point: you cannot infer from a negative statement, that something is not X, that it then must be Y. Of course, if you think about it, that phrase is curiously incapable to do that job, because black and white are no exclusive options either. Not even if something is not white it follows that it must be black: there are different shades of gray, and then of course there are also all sorts of other colors. Even in that field things are not as black and white.
 What Plato refers to here with 'beautiful' isn't quite what the modern word means. That the focus is eros should be ample indication that the instances to discuss should better not be landscapes in nature or urban areas created (and, in this example of ugliness, neglected) by man. What I have in mind in my own account of beauty is of course neither/nor, but again something else, different from both Plato's concept and what is current today. Part of the goals of this blog is to understand the differences between all three views. In any case, the point made here is neutral in this regard: whatever we understand beauty and ugliness to be, they're not simply two extremes on a linear scale of some measurable quantity.