Can I get the exact peak/maximum rate value from IO Graphs

I don't know how exact it has to be for you, but if you need the absolute number of bits/second down to the last digit you probably can't get that from the I/O graph. As far as I know there is no way to "ask" the I/O graph what the highest peak is in absolute numbers.

Question is, why isn't it enough to work with an aproximate value? The I/O graph is usually good enough to get a reading that tells you what you need to know; it is rarely important to get more detailed values that what you can already read from it.