Hasrate History Chart

The hashrate history chart seems to show a lot of downtime. Is this because it is an estimate of hashrate by success-rate?

My output for Scrypt should be ~3.5GH but despite a few high peaks well above that the chart seems to average well below 3.5.

Is that expected based on how chart is drawn or should I be looking for an issue?

All 7 of my machines report full 500MH rate.

Hashrate is calculated by how many shares you find in a 2 hour period.

No matter how fast or slow your miners say they are working, if they are not finding ANY shares they would not have any hashrate.
If every second you have 100% errors, it doesnt matter what speed 500 or 600 or 700 hash the miner reports you are working at.

Essentially it sounds like that is calculating 'effective hashrate' vs 'working hashrate'.

Thanks for your help!