This is post is now quite old and the the information it contains may be out of date or innacurate.
It is entirely possible and easy enough to create all kinds of charts and graphs using only PHP along with the GD library.
These days PHP is generally compiled with the GD library enabled so it is highly likely that you already have access to it. It is a powerful system, unfortunately often only used for creating thumbnail images.
For building management information systems with real time generated graphs there are some excellent classes available which will help you display information in a graphical and therefore easier to understand manner.
Checkout these great free graph classes:
However, sometime the graphics produced by GD simply are not flash enough for your application.
When you want something really flashy on your web site then you really can’t go wrong using flash.
There are a few flash graph packages you can purchase online for not too much cash. However if you prefer to keep it all open source then I highly recommend checking out this open source flash graph class.