This is the rate at which an allowance is used up by employees against a fixed time off allowance for the year. As such it is only useful for Leave profiles with a fixed absence allocation for the year.
The blue line in the chart shows the ideal/target where the amount of annual allowance (not accrual) that has been used up is directly proportional to the time in the leave year. For example, if an employee is entitled to 12 vacation days a year, they would ideally use up 1 day a month which would be represented as a straight line for the year. Actual time taken can then be compared to that.
This can be used to report on employees/teams who use up their allowances too fast/early or too slowly/late.