I have looked into the session mechanism and I can't see how to populate the session variables from a chart drill from many different chart pages into the same drill page. I could set up hidden qSelectors on the drill page to take the values of the passed parameters but when using qSelectors on the same page I have often found them to be out of step with the page they are on (i.e. they don't filter the other WPs on the page until the page is refreshed) when they are used as part of the CAML filter, so I am unconvinced they can be used in the setup I have.