StatsDawg

This graph shows the number of sectors read or written for the disk. Though not equal to bytes, it does show roughly how much data is being read or written. This compliments the I/O Requests Graph rather nicely. The two should provide an idea of how much data is being transferred and how many requests it is taking to do so. This graph may also be helpful in finding bottlenecks in the system, or rather at what times the most data seems to be written or read to and from the disk.