Mhh kay, seems like a sophisticated formula, could somebody post it?
So if a block would have been found in the time, it would have spiked again?
The amount shown is your fixed balance + your income from the current round combined (might be great btw. to have a different colour for the fixed balance in that graph too!)
If you have 50 cents from previous rounds and would earn 10 cents this round, it shows 60 cents. However if this round more and more people join the pool, the pool will find blocks faster - but your own contribution gets lower.
Let's say, the pool hashrate doubles during this round. This means the amount shown falls to 55 cents (if you keep your hashrate constant). It can be expected in the longer run that you will receive these 5 cents twice as fast as the 10 cents though.
As soon as the block has been found, you will see a sharp spike that will even out to the next expected fixed amount after a few shares.