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.
Use of the Google Chart API is subject to a query limit of 50,000 queries per user per day. If you go over this 24-hour limit, the Chart API may stop working for you temporarily. If you continue to exceed this limit, your access to the Chart API may be blocked.
the request to the chart's url will be made by the client browser, which is a unique ip for practically every visitor. so, maybe that's 50,000 charts per day per visitor? unfortunately they don't really clarify what they mean, as they could be tracking the request referrer.
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.