It should be a straight line (in theory) for a zero-order reactant, provided you have used a massive excess of your other reagents (so that they have approximately constant concentrations and so the rate is limited only by the concentration of the limiting reagent).
I suspect the mixture used does not contain a massive excess of hydroxide or acetone and so that's probably why it forms the curve. I'd need to see what the mark scheme says as it does appear that they haven't given you the right information to deduce this.