There are lots of Flash charting tools out there, but when you have clients that have ActiveX disabled and so are unable to run Flash you have to resort to a different solution.
pChart is a PHP class oriented framework designed to create aliased charts. The graphs from pChart are generated using the GD library, so you'll need that installed on your server if you're planning to use this library.
The documentation does mention that creating real time graphs will be pretty CPU intensive so you may want to look at caching or pre-generating the graphs. To help you with this, the pChart framework also comes with pCache which helps in caching graphs - so you don't have to keep rendering graphs on the fly.
And the best thing? It's open source and free (GPL).
To get to know more on pChart and how to use it, head over to their documentation page.
For a look at the graphs you can generate using this library, take a look at the chart gallery.