[ An update on this topic has been posted: Creating Visual Tooltips Anywhere ]

Text tooltips could be a little dull. With visual tooltips, we have more options for data visualization. Drilldown is one of the easy options.

In the Tableau Ambassador 2016 viz, by hovering mouse on a person's picture, you will see a bigger picture and details.
My fellow Tableau Ambassador Rody Zakovich asked me to write about it. I am so obliged.
To illustrate the technique, I came up with a simple example. Please click the picture below to view the interactive version.
Here are the major steps for creating the visual tooltips in the above example:

1.Create a bar chart for the sales in 2011-2014, based on the superstore data set.

2.Create one tooltips sheet for the year 2011, which shows breakdowns in product category.
- Create a filter Year2011 and select True after putting it on the filter shelf.
- Duplicate the sheet 3 times for the years 2012-2014 with some change in the filters: Year=2012,2013,2014 etc. Namely one tooltips sheet per year. Note that the filters are mutually exclusive. Only one of the filters will be true at one time.

3.Start a new dashboard and put the bar chart on it. Remove the title and legends for simplicity.

- Drag each of the tips chart (in Floating mode) to the dashboard and place it near the bar of the same year.

4.Create a dashboard action with filter by hovering. And select the "Exclude All Values" when clearing all the selections.
Voila, we just built a dashboard with visual tooltips. This technique was used in creating this Ambassador viz. I got the inspiration from a VizWiz article on sheet popping.

The tooltips charts don't have to be uniform. Since each tips chart is independent of the others, we can use a different chart for each year. See example 2 in the viz.

Caveats:
1.The data marks under the tooltip sheets won't be sensitive to mouse hovering. So the tooltip sheets have to be carefully positioned.

2.The technique is suitable for dashboards with relatively sparse data marks.

3.The positions of the visual tooltips are fixed.

4.When one data mark is hovered upon, all tooltips' frames will be lighted up briefly. It is a transitional effect. If possible, lay the tooltips sheets on top of each other like in the Ambassador viz, a way to minimize the effect.

5.The regular tooltips may co-exist or they can be turned off.

There have been other options to put visual charts directly within Tableau's tooltips.
http://kb.tableau.com/articles/knowledgebase/barsintooltips
This VizWiz guest post by Rody Zakovich is also a great example:
http://vizwiz.blogspot.com/2016/03/tableau-tip-how-to-create-100-mark-unit.html

In the last Tableau conference in Oct. 2015 at Las Vegas, we have seen a demo on showing charts in the tooltips. So what I got here might be short lived. But it will remain an alternative for creating great dashboards.

For the impatient, you can have it today or even yesterday as shown above.

Last but not the least, I would challenge you to come up with vizzies using the technique described here. Let me know in the comments if you wish.

Enjoy Tableau.