This is a tough one, for the reasons you've stated.
Can you reproduce the data for the graph exactly? If you can, then make sure you write your script so the data is controlled by the script. That way when the graph is displayed, it will be exactly the same every time, in which case you can use a bitmap checkpoint.
For example, if your graph shows the closing price of the Dow Jones Industrial Average, then every day there is a new datapoint on the graph. But if you can specify to only graph from 5/1/2009 through 8/31/2009, then every time you run the test it will graph those same 123 data points for that date range, and the graph will always be the same.
Depending on what you're graphing, that may or may not be possible, but it's the only way I know of to test something as dynamic as a graph.