option for elastic max only, keeping min at zero

[gordonwoodhull]

It's reasonable (and in some situations less deceitful) to want the X and Y axis to stay at zero but allow the upper bounds to be elastic:

http://stackoverflow.com/questions/29708487/can-i-get-elasticx-elasticy-to-only-apply-to-upper-bound-dc-js

Just overriding xAxisMin and yAxisMin sort of works, but that's a pretty ugly solution. I'm not sure what the right interface is here, perhaps just another argument to elasticX and elasticY.