I have two of those and that graph is better than a single drive.
A single drive wil have a max transfer speed of around 140MBs and an av of 104MBs. Min around 80MBs.
Your graph may be distorted slightly by the big dips. These may have been caused by the hard disk doing something else at the time.
A single Barracuda 7200.12 500GB will not give a max read of 200MBs. Nor a average of 177MBs.
Your RAID could go faster at max and I've seen graphs showing a max of around 250-260MBs. It may be limited by the controller.
Is this your system drive? Remember, the drives you see tested are tested as secondary drives, with nothing on them, under the best circumstances. If your OS is on these drives, the readings you see will be lower because of the overhead and other things going on with drives while the test is running, hence the large spikes.
Try benchmarking with CrystalDiskMark or AS SSD; HDtune is not really suitable to test RAID arrays and sometimes gives plain wrong (too low) results. CDM and AS SSD test on the filesystem instead; how other apps would be reading and writing too. This is the only relevant scores in my opinion. HDTune is synthetic because of the way it does I/O; raw single queue I/O while circumventing the filesystem.