It might be easier to work the math on the monthly mileage average figure and draw the graph based on the monthly mpg figure, extending the graph to include the last 12 months. That would allow people to see how their vehicles are affected by changes such as the winter formulations in gasoline.
I'm not sure how you are working the graph so I just figured I'd throw out a possible solution based on fixed numbers.
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2008 Toyota Prius Touring purchased 04/2016 at 101k miles, currently at 186k miles
2009 Acura MDX purchased 12/2019 at 159k miles, currently at 194k miles
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