I didn't notice it in the FAQ, but does the Maximum Reward line in the graph represent the 100% PPS earnings? Should the pool go on a run of faster blocks and clear its payout queue, will the unpaid + everpaid line reach the max reward line?
Yes... And no.
Each time you submit a share, the pool records it in the share log, at it's straight PPS value. When a block is found, 25 BTC worth of shares (starting from the most recent) are paid.
If the pool is unlucky, and it took more shares than average to find the block, some of the shares submitted for that block cannot be paid (since the pool does not have the money to cover them). The pool "remembers" those shares; they are shelved.
If the pool gets lucky, and it took less shares than average to find a block, pool will have some BTC left after paying every share submitted for that block. It will therefore pay whats left to the most recent shelved shares.
However, paying everybody like that in every block would be a problem: First, small miners would only get micropayments in every block; and it would be dust that accumulates in their wallet (and increases their fees when they use the coins). Second, some clients have a bug in which that cannot support a too big amount of outputs per transaction. (Doing payments like that in Eligius would cause more than 5000 output per transaction)
Therefore, Eligius implements a sort of tradeoff. Instead of directly paying the shares when they are found; it internally computes which shares are to be paid, and which shares are not (yet), without actually sending the money to the miner. Instead, it holds onto them until a given amount is accumulated (chosen by the miner), at which time the pool will send the coins.
Therefore, it means that even if the pool were to empty it's payout queue, it does not mean that everybody's shares ever submitted have been paid. It does mean, however, that nobody has accumulated enough rewarded-but-unpaid shares to be paid. (In those cases, the pool sends the block reward to an offline wallet, from which the miners will be paid when the payout queue gets long enough.)
Thus, about the graphs and stats:
The everpaid line is amount of BTC the pool has ever sent to you.
The unpaid+everpaid line is that amount, plus the value of all the shares the pool has rewarded to you, but has not set the actual money yet, because you have not met your treshold. (the unpaid amount is the amount marked in the "unpaid balance" in the table above the graph)
The maximum reward line is the total value of all the shares you ever submitted to the pool, including those it does not have the funds to pay yet.
If the pool gets lucky, "unpaid+everpaid" will get closer to "maximum reward", and may even touch it. However, they would not "meet" at the same time for all users, depending when that user joined. Most recent shelved shares being paid first, the pool would have to be lucky a lot longer to pay all the way back to 2012, when the pool began, than to pay all the shares of somebody who started one month ago.
However, nobody's payment will ever go above "maximum reward": once all your shares have been paid, all the extra coins go to paying other who have to yet been fully paid. Finally even if the pool were to clear it's whole backlog, then it would hold onto the extra coins until less lucky times. (in any case, such event would be extremely unlikely: In the beginning, Eligius suffered from pretty severe bad luck, and thus has accumulated an hefty amount of unpaid shares. According to my calculations, the pool has acumulated over 2800 BTC worth of unpaid shares, which means it would need to find something like 112 blocks RIGHT NOW to pay them all)