after you sending the chart for the very first time to the client,
just include an ETAG or Expire headers to set cache to 1 month ....
so, a same client will only select a new chart in one month :)
* if the chart generation is dynamic, than you are right, you can
cache the generated charts in the server side, mapped to the HTTP
requests (I guess)...
But you should use the server cache carefully.. to avoid the trap of
adopting a comfortable solution of caching things that should be
cached on the client side........