The By drop-down menu lists items used as a basis for a complex sort condition (for example, sorting Cities by the revenue generated in each).
Label—By default, dimensional data items is sorted alphabetically by name when you create your chart: this is equivalent to sorting by label. When selected, labels indicates that the item chosen from the Sort list is sorted by label or name, rather than by reference to corresponding numeric data values in the chart
Value—Sorting by a numeric data item orders each value of the target item chosen from the Sort list by its corresponding numeric value in the Value list.
Sorting by values produces an entirely different sort order. For example, your chart may list each state in which your company has made sales revenue and the total cost-of-sales for each. The states are initially listed in alphabetical order. When you sort by cost-of-goods, the states are ranked in order by each corresponding cost-of-sales figure.