As of this writing, an alert can be set on a continuous numerical axis only. However, there are ideas in the Tableau forum that will increase the flexibility on setting alerts.
In the Tableau ideas forum, users can write down what ideas they have for future releases and the community can vote for them. Ideas with high votes are likely to be picked up soon. You can find the forum here: https://community.tableau.com/community/ideas.
On Tableau Server, in the upper-right corner, you will find the Alert button. Click on it and another window will pop up:
In the new popup, a condition and a threshold can be entered. Also, the frequency ...
