I have always had a dislike for pie charts but was at a loss to explain why. Some of the why might be personal but here are 5 objective reasons that pie charts are sub-optimal when compared to bar charts
1. More difficult on a pie chart to visually sort the sizes from low to high
2. Hard to correlate slices to multiple pieces of information
3. The "other" pie slice can look like the biggest when it is just a grouping of small slices
4. The pie chart is always the same size so though it shows distribution it doesn't show magnitude.
-> If using overall size of pie chart to attempt to show magnitude, this can be misleading - circle comparisons problematic
5. Real estate - pie charts take up a lot of room
Correlation between pie slices and textual descriptions difficult
Size of pie remains the same despite changes in magnitude
Pie charts represent percentages of a whole - for example the spending breakdown, but there is a big difference between spending one dollar and spending a million.
example of Oracle Database home page that uses a pie chart for a breakdown of session states IO, CPU or wait. The problem is the pie chart is always the same size no matter how many users are active.
Limited number of slices can be displayed leading to the "other" slice
An example of pie, bottom right, where "other" is the biggest slice, but actually doens't represent an outlier
Same page laid out with bars showing only the top 4 consumers in each group. This solves the problem of "others" looking problematic and also lines up the text description with the bars.
Beautiful examples of why pie charts are bad
Wikipedia - information on pie charts, including example of the difficulty of ordering