- Link/Unlink charts
- Label outer axes for linked charts
- Compensate gutter in between grouped charts
- TestChartMatrix2.cxx: Replace dynamic_cast<..*>() with
  ::SafeDownCast()
- TestChartMatrix2.cxx: Remove empty lines. Fill up comments
- vtkChartMatrix.cxx: Add //---..
- Reduce render window size.
- Reduce gutter spacing.
- Remove unused chart2 var
- Forward `axis` argument to Unlink and Link
- Remove unnecessary `//@}`
- Use Gutter Compensation To Compute Correct ChartIndex