The Alchemer website demonstrates the use of bar charts versus pie charts. Bar charts are recommended for trend display and comparisons. Pie charts are used when comparing a value against a whole.
In principle, the use of pice charts is discouraged when you want to show precise values or when you want to compare different values with each other. It is also recommended to restrict pie charts to only a few categories. Bar charts can serve as a good way to show comparisons between categories.
It is generally observed that new trends tend to shy away from using pie charts, as it is difficult for the human eye to judge the exact angle that shows the value of a category in a pie chart.