Presentation Rules (e.g. 7x7 in Undergrad Communications Courses): are they supported by evidence?
All axiomatic principles with respect to presentations should be taken with a heavy dose of salt, not just a pinch.

The important issue at hand is that you need to communicate your information to an audience effectively. Putting too much text on a slide makes audiences read the slide, and possibly tune out your elucidations and elaborations of the material on the slide. Similarly, putting too many graphs on the same slide causes the same problem—there's too much to focus on, so you get "lost" in the course of the talk, which again is entirely unhelpful.

Principles such as the "7 x 7" guideline are an attempt to balance between having too much and too little information on a slide. They're useful as rough guidelines, but need not be treated as strict rules to follow.