If you are trying to pull up a historic graph, you will want to use an API call similar to the one below:
http://server/chart.png?graphid=-1&id=555555&avg=3600&sdate=2014-08-27-15-47-00&edate=2014-09-03-15-47-00&clgid=&width=850&height=270
From the url you posted above, it looks like the graphid=-1 is not in the front and isn't -1 but 1 which might be causing the issue.
You can always test this out by running a historical report and then copying the URL for the graph that is displayed there.
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