Hmm. yahoo says IPO was in 1986 and has no prior data...Maybe google provides longer chart than yahoo, isn't it?
So, there must be an error at google...
I think both use adjusted data when charting.However, if I heard correctly, yahoo provides adjusted chart, but google does not.
In Yahoo's historical data there is an additional column named "adjusted price". It is IMO very useful for performance calculations like was done in this thread.