it takes the number of WIP from the last 10 days (one per day) and orders them in ascending order rather than the order of days (2, 3, 3, 4, 4, 5, 5, 5, 8, 8).
For the WIP chart there is only one data point per day so there are 10 dots to use for the calculation. In the Cycle Time Scatterplot there can be many more but it generally works the same.
Then, it finds how many data points are needed from the total to reflect 85% (rounding down). 8.5 data points reflects 85% of 10 data points so it rounds down to 8.
So we find the number in the list we made that is in the 8th position - which, in our case is 5 - and it plots the dot for the trend line for today’s date at 5:
WIP: 2 - 3 - 3 - 4 - 4 - 5 - 5 - 5 - 8 - 8
The same principle applies to 50%, 5 data points reflects 50% od 10 data points so it rounds down to 5.
WIP: 1 - 2 - 3 - 3 - 3 - 3 - 4 - 8 - 8 - 9