You can use the Pie mark to show proportions. Although generally this type of information can be better shown using stacked bar charts, there are cases where pie marks can be very effective—for example, when trying to convey the percentage allocation of marketing expenses by state where the spending of geographically close states are very relevant.
Tableau will never use the Pie mark as an automatic mark type, but you can select Pie from the Mark menu.
When you select the Pie mark type, an additional target named Angle is displayed on the Marks card. The Angle target determines the angular measure of the pie wedges. For example, if you place a measure such as Sales on the Angle target, the 360 degrees of the pie corresponds to the total sum of sales, and each wedge is divided for the values of the field on the Color target.
The view below shows the time it took to ship products by various ship modes. The data overlays a map and shows data by state. We can see that delivery by truck takes the longest to ship in this particular region. That is probably why orders are always shipped by air to the 48858 zip code.
To try some hands-on exercises for pie charts, see Building Pie Charts.