Target Intertrip Idle Time (Idling Query)

The orange line in this Idling query chart includes the target intertrip idle time. Since the target doesn't change, why isn't the target a straight line, similar to targets displayed in other graphs?

The intertrip idle target is a percentage of engine hours that the vehicle idled unnecessarily. In the chart, the orange line represents the calculated intertrip idle hours available for the reporting cycle. As a vehicle's engine hours differ for each cycle, the calculated available intertrip idle time changes, as shown in this calculation:

52 Engine hours

4% X Intertrip idle target

2.08 Available intertrip idle hours

Another cycle may have 48 or 60 engine hours, with a different calculated result.