That is a fantastic statistic.. the fact that it appears to have been positive 90% of the time is a good sign 
The only thing I can think would be maybe to have a % value somehow instead of a btc value, as the btc value will depend on the gross value of the pool (sorry if this doesn't make sense, it's before my coffee time)
The only thing I can think would be maybe to have a % value somehow instead of a btc value, as the btc value will depend on the gross value of the pool (sorry if this doesn't make sense, it's before my coffee time)
Remember that the graph _should_ always be positive while mining, since you keep adding value. If it goes negative, that means coins were exchanged below the _currently_ (not previously) expected value.
Percentage is doable, but I wonder whether it would actually be of any use. Since the percentage would drop over time since you accumulate "paid". (paid has to be considered in the graph since it would otherwise show a huge drop on every payout)
I do have a few ideas for future experimentation, when time allows. (and there are also a few things I dropped after seeing how useless they are in the real world)