Label y-axes

[willymcallister]

Describe the bug
Enhancement: Some y-axes are not labeled with a variable name.

To Reproduce

1. Go to Essence of Calculus article.
2. Search for "A nice way to think of this setup is to draw the graph of"
3. Notice the following graph of "unraveled rings" does not have a label "C" or "circumference" on the vertical axis.

Expected behavior
Graphs should have labeled axes, so they can stand alone with clear meaning. It also helps firmly connect the axis to the surrounding text.

Screenshots