Hey Nico. In 10.0 and earlier, there are no conditional alerting features that come with Tableau Server. However, in 10.1, there is a feature called Conditional Subscriptions that could likely do what you are looking for. And coming in 10.3, which should be released very soon, there is a newer feature called Data Driven Alerts that could also perform what you are looking for. The features are similar but work in slightly different ways. The essential method behind both is that you'll need to build a calc that determines the gap value day over day, and build it into your Filter list in the first case, or into the visualization itself in the latter case.
If you're running something earlier than 10.1, you'll have to go third-party to get this functionality--Metric Insights, a partner of Tableau, can do this, or there's an open-source tool I wrote with Jonathan Drummey called VizAlerts (note: not officially supported by Tableau, does not work with Tableau Online)
Let us know if you have any other questions!
Data driven alerts are available in Tableau 10.3 now. It's available if you have your own server. Does not work with Tableau Online.
Hi all, I want to give a quick update that Data-Driven Alerts is now available for Tableau Online as of this week! Let us know if you have questions on setting up alerts for your use case.