By default when viewing the trade rate for a past transaction it should show it in the most helpful way possible by default. For example if my default or local currency in my settings is set to USD, then all mypast trades should show the rate like this:

1 BTC = 100,000 USD

Instead of like this:

1 USD = 0.00001 BTC

The second way is counterintuitive and requires manual calculations. The first way shows the user what they really care about.