The graph is the only means that we have to determine our earnings. If the pool gave some clear indication of the earnings we would have more to go on. A flat graph would indicate that it wasn't going up during that period. Excuse me for wanting to know what the benefit of me mining on the pool is. I mean if you don't want us to mine on the pool I'll be happy to go elsewhere.
The graph is a terrible way to figure out your earnings. Yes it's nice to see an upwards slope, but really, just write down your "unpaid" value and note the time. Now wait a few hours / days (the longer the better). Now write down your new "unpaid" and divide it by the hours between the 2 readings.
It's relatively trivial to do this with a python script using the zpool API.
I have no idea why the graphs are messing up, but yes it is annoying. In the past it was because of coins devaluing at the exchange before they can be cashed out. ("unpaid" is an estimate, not a set number), I don't know if that's still the case or not, but it happens on all multi-coin pools sometimes.
Also there are problems that seem to coincide with payout processing. I've seen the stratum connection drop fairly regularly during those times, and it's visible as a drop on my hashrate graph until the pool starts running smoothly again. It doesn't happen often in a day, so the overall effect isn't much, but it's not ideal.
Unfortunately, before you "go elsewhere", there are few good alternatives to zpool. One of the major alternatives uses PROP as a payment system and is obviously being gamed by the pool operator for profit. Another major alternative has stupidly high fees (but well hidden in the small print).