Spectral chart

On the chart you can find number of days on the horizontal axis and amount of cards completed for that number of days on the vertical one.

Percentiles

There are 2 red lines on the chart. They are percentiles for the 50% and 85% (you can change that levels by chart options). They mean that 50% (or 85%) of tasks were completed faster than 3 (or 15) days.

So you can provide that information to you customer to predict how many time you need to complete a new task. You can forecast that you finish it in 15 days with 85% probability