According to the release notes Summer '16 (Release Notes) we should be able to create cumulative line charts in Einstein Analytics.
However, I was not able to find any more documentation about this.
Also looking for the
cumulative line chart in the dashboard editor did not reveal a way to create such a chart. (They should only be available in Dashboards and not in Lenses according to the release notes.)
Are there any prerequisites for the source step to show the option to make it a cumulative line chart? Is there some more information on this?
Edit: Unfortunately I've misread the release notes - the linked note is referring to Reports & Dashboards in Lightning, meaning that this answer is a nice way of achieving the desired functionality in Analytics Studio.