If you activate 3 metrics in the visits over time graph, itll fail to load.
I activated visits, bounce rate and avg time on website.
I can find this error on 2 installations from me and the piwik demo.
I use windows 7 and the latest chrome version.
It doesnt matter which php version you use (tested with 5.2 and 5.3).
Dunno which version demo.piwik.org runs.
Fixed in 1.X branch by merging [changeset:056659928c2a6014593d845385c4ad4b9bb690f6].
Moving 1.x closed tickets to 2.0