That interval controls the bucket size of the date histogram. It's not distorting the results, it's just controlling how the data that goes into each sum is broken up.
The problem is that a 1-hour interval creates too many buckets for the visualization to render clearly. The automatic down-scaling is there because that interval size could overload your browser, or just be too fine-grained to get any meaning from it.
You probably need to select a smaller range from the timepicker to get the 1-hour bucket size. Instead of doing "last month", try "last week." If you need a "last month" visualization, you might want to use a daily interval instead of an hourly interval. Note that these numbers are just guesses I've come up with, since I don't know what your timepicker is actually set to.