For these questions, since any particular response isn't exclusive. the pie-chart results are skewed.
Example:
Single question: What ice cream flavor are your two favorites (from a list of vanilla, chocolate, strawberry)?
User 1: vanilla and chocolate
User 2: vanilla and chocolate
User 3: vanilla and strawberry
100% of the users selected vanilla, but if shown in a pie chart, what is shown? 50%? Because 1/2 of ALL responses were vanilla? That is skewed. A bar graph showing a 100%, 67%, 33% distribution conveys much more information.