fix(axis): center axis labels

[marcelgerber]

Fixes #2850.

The issue here is that bounds are the bounds of the whole charting area, but the vertical axis has some extra bottom padding and the horizontal axis has some extra left padding:

V         |
e         |
r         |
t         |
i         |
c         |
a         |
l         |
          |
a         |
x         |
i         |
s         |
          |
-------------------------------------------------------------
PADDING   |                   Horizontal axis

By relying on axis.range, and in particular on the new axis.rangeCenter property, we can circumvent this issue.

[sophiamersmann]

Nice! Thanks for fixing this so quickly! It's one of these things that once you see them, you can never unsee them...