http://www.summitresources.com.au/shares.htm

Displays a couple of charts from a custom graphics engine (the basis is actually imlib2), with antialiased lines and a nice semi-transparent underlay.

http://phirate.exorsus.net/test7.jpeg

Displays a pretty extreme example, this one generated from perl through povray, I had standard, bar and stacked versions of that engine. Disadvantage is its pretty unreadable for real stats, but it looks nice :)

So basically you have a number of questions to answer:

1. Who is the target audience? are they going to require particularly impressive graphics?

2. Are the graphics for general trend information or precise analysis?

3. Are the graphics going to be require in realtime? near realtime? any old time?

4. Do you have a lot of time to muck about with it?

5. Is there going to be a lot of different composits on any given chart (ie, bars and a min/max/median line etc)?

Have a think about those, they should point you in the relevant direction in terms of work required. GD is by far the easiest, but you pay for that in terms of simple graphics. Moving to ImLib2 or OpenGL as your rendering basis makes for harder code but a nicer end result (and often faster). Going to povray/rib is really not recommended (from experience :) :)