So three.js defines cylinders in a way that removes the need for a cone class (you can define cones with their cylinder; by setting the top radius to be zero; http://threejs.org/docs/58/#Reference/Extras.Geometries/CylinderGeometry). I don't like how they do that, but maybe we want to consider matching our shapes to their shapes (Geometry)? There doesn't seem to be a need for us to limit what can be drawn in threejs; right now we would not be able to draw a tapered cone, even though threejs allows it. 

This is a very minor point though, and the extra thought may not be worth the small benefit. Let me know your thoughts though.

If we have a user of pydy-viz who is also familiar with threejs, and they see that we've limited what allow drawing less shapes than threejs allows, that user might think "well why did they do that?". So maybe we should have an answer to that question; otherwise we should allow equal functionality.