Custom graphs let you anchor data and then select a period (month--which goes to 31 days or a full year). Never tried more than a year so I don't know if that is possible.
Or maybe you're talking about custom graphs with scrolling data selected? In that case, you're correct, the max period is 28 days. I seem to remember that was brought up once and warranted a place in the queue of things to look at/consider for future enhancements.