Make VCartesianAxis::get2DAxisMainLine() truly const method.

No internal state gets modified by this method now.

Change-Id: I10475ad3483cb3a92e41208e1a55a6b925c10223
Explain what doAutoScaling() does.

Change-Id: I15ca9ca0c3a22094f0a969adf2b3f1145d116052
Scope level reduction & explain what the max axis index really is.

The concept of axis index and dimension index is quite confusing in the
chart2 code....

Change-Id: I035af0a225396bd5394089d6f584f78dcaf0ad31
More on documenting the methods.

Change-Id: Ibdbc1b26521e56de5dcba1b9572bb0221fc0fff5