There is a typo in the data. The first data entry specifies a different hour from the rest.
Here is a corrected fiddle: http://jsfiddle.net/gordonwoodhull/t5Un3/1/
var data = [
{"id":1,"datetime":"2009-01-26T06:00:00.000Z","peopleVisited":452},
{"id":2,"datetime":"2009-01-26T06:00:00.200Z","peopleVisited":62},
Glad to see that millisecond data does work!