Use boost::optional for these members.

Change-Id: I60c52bf99d0e63e27f7c576e2582c26e19809c8c
Simplify AxisProperties and its surrounding code.

Change-Id: Ie47421d778c2beb8eb8e132532220054f183a5f1
Make thie method consistent with the others.

Change-Id: I028f0dd833cb2307e842805636458f51c71dbd66
TickFactory_2D -> TickFactory2D.

For consistent naming.

Change-Id: Ic94681d4d18c0330ea5a8393c1b3d863c81e1ea8
Make this data member const, as it never gets modified once constructed.

Change-Id: Ic01fd0591bbb524b4a6a0ec1dfe448e33f4d6b18
Re-organize method / class descriptions.

Change-Id: Ib7fe33581c3aec2f58f8a16a07215b69080e0a51
xInverseScaling is set but not used.  Remove it.

Change-Id: I1d185df4b14e152f4da9dfdbc2e71504c2cb1f3b
Improve method descriptions.

Change-Id: I25074578607dedaec9689dff9a6bb9c2fa7be56b
Make it clear which bounding rectangle it returns & const.

Change-Id: I9c8080225cfbe0f38133ce2503bd2d0c1d0f5e13
B2DVector supports operator!=.  Let's use it.

Change-Id: Iecf58e0f24909ec2fd2780a5f4b219105c7c4b47