A pie chart presents data as a simple and easy-to-understand picture. It can be an effective communication tool for even an uninformed audience, because it represents data visually as a fractional part of a whole. Readers or audiences see a data comparison at a glance, enabling them to make an immediate analysis or to understand information quickly. This type of data visualization chart removes the need for readers to examine or measure underlying numbers themselves. You can also manipulate pieces of data in the pie circle to emphasize points you want to make. 

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Bar graphs are used to compare things between different groups or to track changes over time. However, when trying to measure change over time, bar graphs are best when the changes are larger. Bar graphs also clarify trends better than do tables. 

A pie chart becomes less effective if it uses too many pieces of data. For example, a chart with four slices is easy to read; one with more than 10 becomes less so, especially if it contains many similarly sized slices. Adding data labels and numbers may not help here, as they themselves may become crowded and hard to read. This kind of chart only represents one data set -- you need a series to compare multiple sets. This may make it more difficult for readers to analyze and assimilate information quickly. Comparing data slices in a circle also has its problems, because the reader has to factor in angles and compare non-adjacent slices. Data manipulation within the chart's design may lead readers to draw inaccurate conclusions or to make decisions based on visual impact rather than data analysis.