Comparing with the numbers that appear when you hover over the graphical chart, I believe the four numbers are [open, high, low, close].
Also, the data ends with this: "start":1279424586,"period":86400
86400 seconds is 1 day. 1279424586 is Sun, 18 Jul 2010 03:43:06 GMT (http://www.onlineconversion.com/unix_time.htm
). I take that to mean the end of the July 17 trading day in Mt. Gox's time zone. July 17 is also the date Mt. Gox was announced (http://bitcointalk.org/index.php?topic=444.0
), so I'm guessing that is where the data starts.
We still should have a more official (and complete) source rather than reverse engineering the site.