That's exactly what I am doing, I started first as a graphic designer (print mostly) then learned a bit of web (far enough to be able to work with developers) and now I'm more experienced than any of those two in 3D (technical modeling and rendering).
In the company I'm working in, on 15 people only two know 3D including me and only 5-6 know more than indesign/photoshop. And those are the ones that will never loose their job!
Being able to use multiple tools and combining them is the best method. Lately I've done some 3D renders in Cinema 4D for a water ripple effect that I used on packaging for medicals, I'm both able to do the whole pack (diecut included, with the right spot colors and typography) as well as the pack artwork.
I'm currently updating my portfolio: http://www.shapenoid.com/protected (pass: stojan2014)
You have two choices, either you become a master in 1 domain and clients hire you for that, or you become good in multiple domains and any agency would hire you. Depends on what type of work you want (solo or team).
Is there some sort of T graph generator that is used in the article, or is that just a made up visual?
Those look like GitHub indenticons: https://github.com/blog/1586-identicons
Similar, but as far as I can tell, those avatars don't correlate to your skills.