What is the term used to represent by means of a diagram or graph, including relevant features?

The term that best represents the action of using a diagram or graph to show relevant features is "illustrate." This term encompasses the idea of providing a visual representation, which often includes diagrams, graphs, or images, to clarify or enhance understanding of information.

When you illustrate something, you not only depict it but also convey essential details that help others grasp the concept being presented. This is particularly useful in educational contexts, where visual aids can significantly enhance comprehension.

On the other hand, while "sketch" can imply a rough drawing, it does not necessarily incorporate the level of detail associated with highlighting relevant features as effectively as "illustrate" does. "Simplify" refers to making something less complex, which does not inherently involve visual representation. "Outline" means to give a general description or a summary, but it does not capture the visual aspect that "illustrate" entails. Thus, "illustrate" is the most fitting term for the described action.