thick plate like that is tough, the score line has to be pretty deep, and the pressure to break it stright needs a "brake" rig. If it is safty glass, like some new showcases are made from it will shatter if you try to cut it, you need a special cutter to do it, BUT any one who sells glass will do it.
if the plate glass is scratched up... it will be even harder to cut without a shatter.
you can buy a diamon wire saw that cuts glass, they are expensive and SLOW, but the do make a nice cut.
Very old rolled glass thats brittle will snap easy even at that thickness, sacrifice a small chunk and experiment with scoring snapping it. I never could get the hang of it myself, i have my buddy who owns the hardware store cut any cglass I need... he aint much better but he has the cutting rig.