Thanks for forrestv's answering!
Yes, I've change my miners' username from "miner1" to "1NxxxxxxxxUYS" (a Bitcoin Address).
I really don't know that the "current payout" only display the payout of the default address. It's confusing, because I won't know the overall performance of my p2pool node.
Could p2pool seperate the graph to show the overall performance and individual miner's performance?
I have 2 questions:
1. Why did it go down?
2. Will it go up? Why?
Are you mining to a different address by setting your miner's username to a Bitcoin address? The "current payout" graph only displays bitcoins going to the default address. Did you maybe change your miner's username to a Bitcoin address about 24 hours ago..?