The ECO level bar graph is the answer to your question about the trip mileage. It simply indicates the current trip mileage (since powering the car up). The chart starts at 25 mpg, and ends at about 42mpg. If your trip mileage is greater than 42mpg, it will just show the graph completely full.

When you first start out on a trip, it typically won't show anything right away (you have to use gas when first starting out, and your mileage during startup is probably only 10-15mpg).