add two failing ordinal bar tests
add test for changing number of bars
tweak documentation, fix link
add a couple elasticX tests
fix elasticX and watch manual domain changes

record current domain and compare, do rescale if changed
stack mixin was filtering on current domain even when elasticX
reread ordinal domain when elasticX as well (elastic ordinal)
fix ordinal bar charts

it was the outer padding.
also, set _gap undefined when using padding instead
(start to think about gap versus padding really being modes)

fixes #533