Passing Multivalue Parameters Through A Report Link

I've created a line chart in my report which lists number of transactions by month. I've also created a report to list transactions by day. My goal is for the user to select a month and link the report to the graph with the selected month divided up into days (in other words, the user wants to see the number of transactions for each day in the selected month).

Anyway, everything is working perfectly except for one thing. In the list of parameters that I am passing to the report for each day, I want to pass a multivalue parameter which contains all the transaction IDs the user selected in creating the report (these are supplied by a multivalue parameter). However, in the parameter dialog box where it asks for a value to send to the awaiting parameter, I do not know how to supply more than one value. If I need to pass one value, it will work fine. However, I would like to do something like:

JOIN(Parameters!TransactionID.value, ",")

for the value of TransactionID, but when I generate the report it is not accepting the values. I'm pretty sure its just a format issue, and I just need to know how exactly I should pass these values.

I already understand that when passing multivalue parameters in a URL you need something like:

TransactionID=1&TransactionID=2

...in order to select multiple values. However, this is a slightly different situation. I'm really running out of ideas, so any help would be much appreciated.

Thank you guys so much