Add my vote to this feature request as well. I like to show multiple discounts on a single invoice, so I can show exactly what is being discounted from the previous Time Entry line. I would love to be able to add negative Time Entries, or % discounts as a line item in Time Entries, instead of a single discount.
Right now I create Payment Credits for each discount but this is time consuming and doesn't show up on the invoice in date order. The discounts/credit show up at the bottom, and I have to take the time to enter what Time Entry description this credit applies to.