Basically, there is no 10,000 request limit. but, even if there was one, it would still be very easy to cache 10,000 unique graphs on the server, and then serve them up without making new requests to the chart API. Here's an excerpt from its Terms of use:

4.5 You acknowledge and agree that while Google may not currently have set a fixed upper limit on the number of transmissions you may send or receive through the Services or on the amount of storage space used for the provision of any Services, such fixed upper limits may be set by Google at any time, at Google's discretion.