Diagnose disk I/O per application
The tools are in the terminal app:


fs_usage shows the io on a per file instance and grep will help you filter results to one app if desired. 
vm_stat 10 shows aggregate swap activity every 10 seconds - you can fiddle with the timing based on how close you care to look


If you want a nice graph and full monitor, check out Instruments in Xcode. It has amazingly detailed documentation on how to profile, but it basically uses the same terminal commands to get it's raw data but has tons of ways to inspect, view and measure what is happening. Think of it as Activity Monitor Pro (I'd say Pro^10 if you are math inclined :-)