Warning! This post is barely finished. I putting it up as a reminder to myself that I had a hard time finding a post explaining how to do it and I should write a post on it.
"It's impossible" they said. "It can't be done!" they claimed. "You're mad (but still surprisingly handsome!)" the women screamed. To tell the truth, no one said any of these things, although they all ran through my head at various points.
I had a stacked bar chart in SSRS (SQL Server Reporting Services). Some of the series within the bar were so small it was virtually impossible to make the tool tip show up to display their value. What I needed was a way to make each of the tooltips display all of the values for the entire bar. After much hacking and then some more hacking I managed to put something together. The keys to the whole thing are the following expressions functions: LookupSet and Join.
After running across Dustin Ryan's BIDN.com post LookUp and LookUpSet Functions New in SSRS 2008 R2 I was able to cobble something together. The expression for the tooltip on the data point is as follows:
=join(lookupset(Fields!CustomerType.Value , Fields!CustomerType.Value , Fields!FavoriteBrand.Value , "ReportAnalytics"),"" + vbcrlf)
I'll quickly explain what a few of these values represent: Think of the first "Fields!CustomerType.Value" as being the value that is external to the array. It is the value being passed in to the LookUpSet to limit it from showing everything. The second "Fields!CustomerType.Value" is the column in the array which the external value will join on. The third value "Fields!FavoriteBrand.Value" is what will be displayed in the tooltip. "ReportAnalytics" is the name of the report's dataset itself. The final ","" + vbcrlf)" is part of the JOIN which format the array.
Good luck and let me know if it works for you!