@Zapp I think you are misinterpreting the graph. The grouping by hour is showing you how many minutes a station ran that started at a specific hour. In other words, the x-axis is hour 0-23 and each hour is how many minutes it ran after starting on that hour. Grouping by none shows you what you want I believe which is a function of time. Grouping by month is similar to hour but shows you how many minutes a station ran in that month. DOW is similar to both hour and month but its day of the week (Sun-Sat).
Hope that helps!