An interesting topic came up during an email exchange with a student in the course, and I wanted to share it because some of you might be thinking along similar lines.
The question is: Can you and should you use PI Web API to extend PI Vision?
This is a big can of worms: there is actually another complete online course about the PI Vision Extensibility framework! If you are interested in making custom PI Vision symbols, please take a look at this other course.
The short answer is that yes, you can use PI Web API in a custom Vision symbol. But the Vision extensibility framework actually provides an even easier way to automatically consume data updates in a number of common "DataShapes" like single values, timeseries data, etc. Using one of the built-in DataShapes is preferable to calling PI Web API -- but if you need to do something you can't do easily with the built-in options, you can always use jQuery to send a request to the PI Web API using the material from this class, and get whatever data you need.