Steve you are correct, this is not accessible. There's an Idea out there you can try search for and voting up. --Shawn
Tableau neither supports tooltips in field headers nor in individual dimension items.
The good news is that they are watchful to ideas and their comments and score.
Therefore please consider up-voting following ideas:
Scope_ Idea Score _Created Field Tooltips in headers (field labels, quick filters, parameters, etc) 038 _2013-05-22 Item Tooltips in individual dimension items 005 _2014-09-23
See more suggestions in the Tooltips ideas collection.
Just upvoted both of these ideas. They are good ones! Tableau's certainly been upping their game in the tooltip arena, this would help keep the tooltip excellence moving forward!
Thank you! We agree!
Such tooltips would make Tableau dashboards much more useful for dashboard consumers, especially those who had no part in its creation and thus probably have more difficulties understanding it.
Dashboard authors have of course less (if any) need of such tooltips because they are (supposed to be) intimately familiar with data used in dashboards they author.