Try clicking on the gears icon on the My Logbook page and change it there. If it looks like it's already been set you might need to force refresh the My Graphs page - Crtl+F5 on PC or CMD+Shift+R on Mac Chrome.
Edit: Actually, I just looked at my Graphs page and your Graphs page and mine is Font grades and yours is V grades. I suspect it's choosing the grade type your you've done the most of as the display grade. I'llI'll to double check the code responsible for this tomorrow as I'm travelling at the mo.