How to add parameters with mulitselection enabled in ssrs 2008

Well,

I have a table called Resource (ResourceId, ResourceName) and a Session table (SessionId, StartDateTime, EndDateTime, ResourceId)

I have a line chart that shows the number of sessions in a given month for each resource. so it will be something like

Count Month resourceid resourcename Jan 10, 2006 2 ResourceA Jan 9, 2006 1 ResourceB Feb 3, 2006 2 ResourceA Feb 19, 2006 1 ResourceB

So the question is, are there more resources. I create a new parameter for the report and bind it to the resource table so that the parameter has a drop down list of resources. The problem, however, is that I can only select one resource, it cannot be more than one. How can I not select ResourceA and ResourceC to see a comparison of their sessions.

The question is if I have a parameter for the report that can select multiple values, i.e. can select ResourceA and ResourceC and it shows me a line chart for those resources, then if I select ResourceD and ResourceX it shows me the line for this.

Hope it helps.

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