As with all the debt payoff charts, TRACK YOUR BALANCE, not your payments. This chart shows your progress in reducing the balance to 0, it will not show you a total of how much you paid including interest.
After each payment clears (whether your regular payment or extra to the principle) check your new balance and color in the appropriate amount of spaces.
You can color in half spaces if you want to.
You can print out a chart for every year and fill it in for the amount you want to pay off that year. This breaks down a many year process into smaller bites so you can see your progress.