Several minor improvements and bug fixes

Surface:
  Improve title offsets.

Graph:
  Disable edge min length (may revert later).

MultiChartSurface:
  Restrict table contents to label and weight.
  Common up switch code.
  Allow default chart to be nominated.

XYAxis:
  Format weights sensibly.

SVGWidget:
  getBBox add rounding option.

Marshaller:
  Always use 2DChart, but default to nominated view.

Signed-off-by: Gordon Smith <gordon.smith@lexisnexis.com>