It would be interesting to see the prices up here in Canada and in the northern US. While the prices probably swing a lot more I suspect the base line might be a bit higher because they get a lot less mileage on them
Where are you getting your values and prices? If you are using the NADA book to determine values, keep in mind that it will lag actual sale prices, skewing values to a later date. This would explain the Aug-Sep "hump" since it would be reactionary to actual sale prices from Jun-Jul.
It would be interesting to see the same data for another sports car or convertible.