How to display a value in the Tooltip field of a graph.

I have a graph report with monthly cost by cost_date from a dataset called UnitCostTeam. I would like to display a value from another dataset call Comments, the values name is Comments, but I need the comments to match based on where cost_date = CommentDate. So when you hover over the graph point for Jan 14, the comment for that month (CommentDate) is displayed and when you hover over the graph point for Feb 14, the comment for that month is display.

I tried an expression
=IIF (Fields!cost_date.Value = First(Fields!ReportDate.Value, "Comments"), First(Fields!Comments.Value, "Comments"), "no")
But that just displays the first comment and every other month displays no.

thanks.

Paul