While the Trends tab allows the selection of a Custom time interval, the Resources tab does not. It only supports a set of "Last x" selections.
Here's a pair of one-week charts from the Memory tab of Resources for one of my servers:
I'd like to be able to focus in on a 24 hour view of December 14, and then maybe an hourly view.
For another of my servers, I see this:
Something interesting apparently happened between 3pm and 4pm on December 9 and I'd like to be able to focus in on that period to see what other charts might show something interesting that might be smoothed out too much to see in the week view.