In the Chart inspector, click Series, and then click Advanced at the bottom of the Inspector window. Then click Error Bars.

For scatter plots, you can have error bars for both data series. Set them separately by following the instructions for both the axes.

In the pop-up menu, select the way you want the error bars to be displayed:

• Positive and Negative:  
  Displays full error bars, both above and below each data point.
• Positive Only:  
  Displays only the part of each error bar that falls above its data point.
• Negative Only:  
  Displays only the part of each error bar that falls below its data point.

In the second pop-up menu, select the kind of error bar you want to display:

• Fixed Value:  
  Displays an error bar of the same absolute value for every data point. Specify a value in the adjacent field.
• Percentage:  
  Displays error bars based on a fixed percentage of each data point value. Specify a percentage in the adjacent field.
• Standard Deviation:  
  Displays error bars based on the standard deviation of your data set. Specify the number of standard deviations in the adjacent field.
• Standard Error:  
  Displays standard error bars.
• Custom:  
  Lets you set error bars based on your own criteria. In the Positive field, specify how far above the data points you want the error bars to extend; in the Negative field, specify how far below the data points you want the error bars to extend.