Hi Quinten,
The x axis is not reversed. I believe the reason for the confusion is that the direction of the applied linear period chirp is the opposite of what you might expect. The equation OptiGrating uses for linear chirp is Lambda(z) = Lamba0 – Delta*(z-L/2)/L, where Delta is the specified chirp rate (Eq. 21 in the OptiGrating User Reference). If you set the chirp parameter to a positive number, the period will decrease along the length of the grating instead of increase. If you set it to a negative number (-10 in your case), the period will increase along the length of the grating, and if you leave everything else the same, the effect will be reversed, as you expected (the dip you saw in reflection becomes a peak, and the peak in transmission becomes a dip, see images below).
Amine