When the range labels overlap the separator string is ' - '.
The problem is that this character can be interpreted as 'minus'.

Steps to reproduce

1. slider range with high value just above 0
   http://jsfiddle.net/04sr3ty0/1/

Expected behaviour

Should be able to tell if the high value is positive of negative

Actual behaviour

Looks like the range is -2 to -1 when in fact it is -2 to +1

I am happy to create a pull request to fix this minor issue