Error bars need a stated meaning before they can help a reader. A fixed tolerance, a percentage range and a statistical estimate are different assertions. Obtain the intended method and its justification from the person responsible for the data before making the chart look finished.

Record the units, the rule and whether the range extends above, below or on both sides of each plotted value. If nobody can explain where the numbers came from, leave the question unresolved rather than adding a familiar-looking bar as a visual convention.

In Numbers on Mac, select a compatible chart and open Format, Series, Error Bars. Choose the error-bar type and calculation option, such as fixed values or percentages. Scatter and bubble charts provide separate X and Y controls. Styling applies to the selected series. Auto-Fit is enabled when a chart is created; clear it when you need to see all error bars.

Use a known example to check the chosen rule

For an illustrative practice chart, suppose the documented rule is a fixed plus-or-minus 1 unit around values 5 and 20. The corresponding endpoints are 4 and 6, then 19 and 21. The absolute extension is the same for both values.

A 10 percent rule would instead extend by 0.5 around 5 and by 2 around 20. These are different ranges. Compare the visible bars with the rule you intended and inspect the settings if the pattern resembles the wrong example.

The examples demonstrate arithmetic only. They do not establish that either rule is appropriate for your observations. A range should be labelled with its real definition, and a confidence claim requires the analysis that supports that claim.

Check the series and the finished chart

When the chart has several series, inspect each one separately. Confirm that the correct rule and axis have been applied, especially when the series use different measurement units or methods. Adjusting one visible bar does not justify assuming every series is configured identically.

View the chart at its intended delivery size. Check that the bars can be seen, that crowded points do not conceal the relevant range and that the legend or caption explains the method. Keep the source calculation or documented tolerance beside the report so a later editor can update the chart without inventing a new interpretation from its appearance.

Reference: Apple: chart markings and error bars in Numbers.