[jakevoytko]

Your junior colleague is trying to solve a few problems at once. This is overwhelming them, and likely frustrating them. They're trying to simultaneously solve, "Why do I need to represent data visually?", "What data am I trying to present?", "How can I represent this visually?", "How do I map these things in code to this thing on paper?" without understanding any pre-existing solutions to these questions.

In comparison, you have a mapping from "I have a problem" to "this type of diagram will help me solve it."

At my first job out of college, my boss was obsessed with diagrams. He made sure that every person on the project could reproduce the system diagram. This means that I was personally given multiple years of instruction on how to visually represent data by someone who was very patiently correcting me. Are you willing to give someone that type of attention?

If you want them to learn the skill in a useful time horizon, and they're willing to improve, you're going to have to practice with them. Pick a thing they know well. Walk through an explanation of the system with them as you diagram it. Then have them draw the system while explaining it and having access to your diagram. Then have them reproduce the diagram without looking at it. This is the beginning - you'll need to do this a lot before they start to internalize a thing you've practiced for years. Over time, you'll realize that you can leave out some of these steps, and over time, they'll have enough bootstrapping that they can produce novel types of diagrams themselves.