Interesting graph.
In my company's anecdotal experience DDG referrals have been historically flat. They went up a little bit around 1 year ago, then settled back down to their consistent (and relatively small) production. They rank 12th in our list of referrals, or 0.24% of total.
Bing is 2%.
Google is 47%.
(Good ol' Yahoo is 2x DDG)