Overall, I think the decision to use a bar or pie chart is a personal choice. If you are using graphs as part of a presentation, focus on the overall story you are trying to share with the graphical charts and images.
Below is the abbreviated guideline I use in evaluating whether to use a bar or pie chart:
- Bar Chart when noting trended performance (e.g., say, over time)
- Pie Chart when showing distribution of the whole
Let's say you want to track how you spend your money. And this month you spent $1,000. If you want to illustrate how you spent the $1,000 by category (e.g., food, clothing, gasoline), then a pie chart may make the most sense. However, if you want to display the trend for monthly spend over time (e.g., the past 6 months), then a bar chart may be more optimal.