Add an attribute to the chart (or img?) tag to allow a user to specify a chart as "single use". A chart with this attribute set will have its chart definition removed from the session after the first time it is requested.
The implementation for this can likely be a query string param that gets generated and sent to the cewolf servlet. Since the servlet is responsible for getting the chart definition in order to render the chart it can also remove the chart definition once it is done with it.