Should be a way to do this. Could you share your workbook?
You don't necessarily need to use a reference line for the label. One option is to place both labels (the difference and %) on your second bar - the one labeled "Not New".
For the difference calculation you can do this:
if attr([New or Not]) = "Not New" then lookup(avg([Volume]),-1) - avg([Volume)]) end
For the % calculation you can use this:
if attr([New or Not]) = "Not New" then lookup(avg([Volume]),-1) / avg([Volume)]) end
You can place both of those on your Label mark. Make sure they're both calculating using Table across. Then you can format them however you'd like from there.