I noticed yesterday that Mt Gox was no longer listed at bitcoincharts.com because mtgox.com site was no longer actively allowing exchanges in the previous 24 hours and therefore the site didn't have any data, and the volume was 0. However, since then, tcatm had implemented a work-around specifically for mtgox to preserve its 24 hour volume data to appear as ... well, it's not real-time, so, I have no idea why the data is preserved at the site...other than maybe to help promote mtgox perhaps due to a vested interest maybe? no idea...
Anyway, Bitcoin Charts seems to be a mix of real-time and historical data, but currently only noticeable with bias towards mtgox. I haven't monitored or noticed the data for other exchanges to determine if the data is real-time, historical or modified/manipulated in any fashion.