CURIOSITY seems to be the horizontal line; i.e. the person's latent curiosity about things, domains, areas, etc, and the urge to explore/discover is what drives the horizontal movement.
The 'amount of paint' variable, which is clearly the most important part of the physical reality underpinning the metaphor, would perhaps be akin to COMPETENCE. Competence is a stand-in word for intelligence, grit, suitable background knowledge, and probably a host of other qualities.
So what's the grand conclusion here? That highly curious and highly competent people tend to be prolific in their output?
Not so surprising perhaps, but fun to think about.