Pie charts are useful for visually comparing the percentage of events that have occurred, such as the type of error that occurs the most in your system. A pie chart will compare different values for the same field. If you want to compare values for different fields consider using a histogram.
To create a pie chart, use an aggregate query that provides at least a few results in the Aggregates tab.
For example, you'd use the following simple query to get results from your logs:
(error OR fail*) AND exception | count by _sourceCategory | sort by _count
which would produce results such as:
Create a pie chart
Create or open a Dashboard (New) and click on Add Panel > Categorical.
Provide a Log or Metric query and press Enter for it to run.
Once the query runs you will need to flip the chart type to Pie.
Modify the chart as desired.
Click the Add to Dashboard button on the top right of the window to add the panel to your dashboard.