add visible bars for bands with nothing but zero-values

biserver-1711
Merge remote-tracking branch 'origin/dots-ortho-radius' into transition

DCL: Also, changed the implementation to:
* provide a property to control the minimum bar height
* make zero bars play nicer with the stacked margin (by centering zero bars in that margin gap).
* by showing the min-bar even if other bars of the same band are non-zero.

Conflicts:
	protovis-d3.3.js
	protovis-r3.3.js