Why are you doing that with JavaScript?
Because it's neat, and I'm bored. A more real reason could be shifting the load from your server (ie building and displaying graphs with JPGraph) on to the client machine, where there's far more resources available. Say your server has to show a graph for 100,000 site visitors. Compare your 1 server generating all 100,000 graphs, which you may not be able to cache, with 100,000 client computers generating them individually. End result - far less load on your server, hence it can cope with far more views and/or respond quicker.
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Richard Heyes