Basically my graph is just three columns of data. The first is the monthly value of the car. I started with what I paid for the car ($98700) and applied an approximate monthly depreciation factor of 1.4% (I forget where I got this but I got it from somewhere, it annualizes to 15.6% a year). Each month's car value is 98.6% of the previous month's value. This is of course not very accurate to start as in the first year of car ownership there are some big factors such as the federal rebate and faster than average depreciation but this formula should be a good approximation after a few years. You can tweak the factor if you like when you get more data, like blue book values.
Then the next column is just the monthly remaining principal of the loan. Easiest way to get this is to just use an online calculator to calculate your monthly payment, then reduce the prior month's principal by that amount, and add back the monthly interest from the prior principal.
The third column is just the difference between the first two columns, equity (assets - liabilities, car value - loan value).
As far as how much of a down payment you should make, if you are getting a 1.49% rate I'd minimize it. There are a few other considerations. If you are plannign on taking out a big loan, like a house loan, the bank will car about your monthly debt payments. If you are stretching on the home loan you'll want to minimize the size of your car payment.
Also, if you don't have gap insurance from your lender or car insurance company, you want a safe margin where you can feel certain that in the event of a total loss, the value of your car as determined by your insurance company will be greater than your loan. In my case, I felt like $13k was enough of a buffer. Progressive came through for my in my total loss this year and paid above what I expected but every insurance company is different. If you have an insurance comapny that guarantees replacement value as opposed to actual cash value, you can worry about this less as well.
