Lspace is a great site for Terry Pratchett and Discworld info, I've just never seen this chart before, me LIKEY!AboveUp said:This might help:
Most series the order isn't extremely important, with the exception of the Watch Series. The Watch revolves around a small group of guards who start out as a small group of losers and end up becoming the main military force of the city. You'll slowly see them grow and gain new members, that's why I think that series the order is quite important.
If you're looking for a good book to start on, I'd suggest either Mort or Small Gods.