You also need the graph to show at least to -12dB, so you can see the flat and -3db response. SPL in WinISD is not what it will be in your car and you don't need to see anything above about 35dB.
WinISD models assuming an anechoic chamber, not a vehicle or room... so your real world performance will differ. That said, a box with 6dB of acoustical gain might well do 140dB in the vehicle.
Also, check that the TS numbers you entered are in the same measurements as those published by the manufacturer (liters, feet, etc.), because I've never seen a graph like those and I've been using WinISD Beta for about ten years.