Rates are going to have to be calculated by your calling application. All of the statistics interfaces (in the case of pools it's with the LocalLB::Pool::get_statistics() method) are returned as either counters our totals. To turn counters into rates (ie, bps), you need to take a sample of two points and divide by the interval between the two (c2 - c1)/(t2 - t1). BTW, we've included the timestamp the data was sampled on in the returned statistics data structure.
We do store some rate data in our performance counters and that data is available in csv format. This data is what is behind the performance graphs in the GUI. This isn't customizable and is total throughput across all of the virtuals which is probably not granular enough for what you are looking for. Take a look at the "Overview.Performance" menu item in the GUI for the graphs of that data. The methods for retrieving this CSV data is in the System::Statistics interface.
Hope this helps...
-Joe