The line graph helps you visualize the last few fuel-ups, and the bar graphs help you visualize the last few months of data. You don't have any calculated fuel economy yet because you have a partial in there. When you add a partial you don't get an individual fuel economy calculation for that fuel-up or the next. (Here's more info about partial fuel-ups.)
Once you have a few full fuel-ups in a row, you'll see a line graph. Once you have at least three months of calculated fuel-ups you'll see a monthly bar graph.