How to read this chart
With the default linear method, Q1 is 4, the median is 5 and Q3 is 7. The IQR is therefore 3. The lower fence is −0.5 and the upper fence is 11.5, calculated as 4 − 1.5 × 3 and 7 + 1.5 × 3.
The smallest and largest observations inside the fences are 2 and 8, so these are the whisker endpoints. The value 30 appears beyond the upper whisker. The maximum in the summary remains 30: a flagged observation is still part of the dataset.
Try the calculation yourself
Switch the whisker rule to minimum-to-maximum to see the upper whisker reach 30. This changes the display convention without editing the source values. Keep the selected convention in the explanation so readers do not interpret two different box plots as different measurements.
Open this exact example, change the data or calculation settings, and compare the result. The CSV preserves the input rows; the ECharts JSON contains drawing options.
What this example cannot tell you
An observation beyond a fence is not automatically an error or evidence of a separate population. The rule is a descriptive flag. This invented sample supplies no collection context with which to justify exclusion.
Why is the upper whisker 8 rather than 11.5?
The fence is a calculated cutoff, while this whisker must end on an observed value within that cutoff. There is no observation at 11.5. The largest eligible observation is 8, and the next value, 30, exceeds the fence.
Calculation convention
The default uses linear interpolation at rank (n − 1)p in the sorted sample, equivalent to R quantile type 7. At p = 0.25, 0.5 and 0.75 this gives Q1, the median and Q3. The alternate method takes medians of the lower and upper halves, excluding the middle observation when the sample size is odd. These methods can give different results for the same small sample.
Method background: NIST: box plot and fence interpretation. R documentation: quantile methods.
Created by SupaMakers for ChartsAI · September 8, 2026. Source and reuse terms.
