From The Document Foundation Wiki
In addition to the colors you can either keep the calculation Automatic (varies from the minimum value to the maximum value) or choose the method of calculation:
- A percentile is each of the 99 values which divide the sorted data in 100 equal parts, so that each part represents 1/100 of the sample population. A percentile return the value for a data series going from the smallest to the largest value in a set of data. For Alpha = 25, the percentile means the first quartile = 50 Alpha is the MEDIAN.