ive used pCharts for strictly php.
http://pchart.sourceforge.net/
It doesnt do real time updating since it spits out an image. Im sure theres a slick way to do it but for what i was using it was quite minimal
Open Flash Charts is slick. i dig it.
http://teethgrinder.co.uk/open-flash-chart/
I switched from pCharts to OFC for a project i was using. I like it much better.
I tried google charts but i find it lacking. It is real hard to control and just not quite polished for what i was looking for. That and (i think) it requires an internet connection to dial out to google. Its potentially a problem if you're actually building thats not a webapp.
Also someone announced a js based charting engine here on ycombinator about a few weeks back. I thought it was real slick... i wish i had it book marked.