since this chart goes down as well as up it can only be interpreted in a way that it isnt all-time but unique addresses used within a certain time frame - for blockchain.info thats very likely 24hours.
My interpretation was "used" means "hold coins".
Looking at "all time" and hovering near the beginning, you get 1 and then 0, so I guess fornit is right. Gnah. I would love to see much more text for almost all the graphs at least to tell what they are but also to maybe tell what the author thinks they are good for.
Edit: Ok, then where is the chart that shows what I thought this chart would show?