There is a request in the IDEAS forum for this.
Up-vote it here:
Really the best way to achieve what you need to do until this gets implemented is to hack up a sheet to look like a legend, and then use colored shapes to make your "legend" shapes.
Please refer my website for similar types of tricks:10 Tableau Timesavers - Tips and Tricks (Part 2) - TableauLearners TableauLearners
Creating a custom legend with shapes
Formatting is an important part of both your analysis and presentation. You can format almost everything you see on a worksheet including the fonts, shading, alignment, borders, and graph lines but sometimes tableau default behavior is a hurdle. As we all know, the default color legend in Tableau uses small squares. In many case, this can be proved as showstopper for your dashboard and even confusing if you are using other shapes in your chart. For example, if you are using a dot plot. The color legend will show squares, but every other shape in the visualization is a dot. In cases like this, Square shape doesn’t make any sense. So for getting the desired result, follow the below steps
Create a new worksheet with the dimension you want in the legend and use custom shapes to create your own legend. If you are using a Tableau 9.2 Version then you can use a very interesting feature "Match Mark Color” which is found on the Font drop down in the Label menu and you can also change the colors of the text to match the shapes. Then drop that sheet into your dashboard in place of the normal color legend and see the magic. For More Information, refer here
But what if you are still using lower version of Tableau say Tableau 8.3 or 9.0.
Follow the Same process as described above like Create a new worksheet with the dimension you want in the legend and use custom shapes and assigned a common shape say circle to the dimension. Do some formatting like change the “Row Divider” and “Column Divider” to “None” present in the Format border. Also format the tooltip in such way that it should only show dimension name as below.
Then drop that sheet into your dashboard (keep it Floating), adjust the width of the sheet and put this new sheet on top of Shape legends something like below.