Hi Michael. I have Android and if I click on the graph then a new page is opened and the workout details are shown.
This feature should be available for IOS according to this release note: Zwift Companion Release: August 1 (2.4.0 iOS, Android)
Best thing would be to check if you have the latest app version and then contact Zwift Support if this is a IOS related problem and by when they will fix / make it available.