I don't know of a wireframe tool specific to visualizations, but when I am experimenting, I usually turn to R, which has an incredibly wide range of visualization libraries, and is open source.
I tend to think of visualization tools as complementary to predictive analytics. They help in situations where there is not yet enough understanding to build a predictive model, and rely on human interpretation to notice "unusual" data elements. But they can also be used to provide context to a prediction or a detection process.
R and Shiny depend on Python, which is one reason they haven't been considered for integration with Moodle core-- we don't like to add dependencies to Core features. I wonder if a Shiny visualization plugin for Moodle would be of interest?