This chart shows trend for lead time. If it grows it means that your velocity is going down or tasks became bigger.
By default that chart is builded for 85% percentile (for 85% of the fastest tasks).
For example, we want to view statistics on tasks on the programmers board. We select a range of columns and tracks, excluding urgent tasks, so that they do not spoil the statistics.
Further, the system offers to select a date and period. The period allows you to divide the selected above date range by the specified number of months. The choice of the period will depend on how quickly the team manages to complete 85% of the tasks. For example, if this time is equal to 30 days, then you should not choose a period per month, otherwise the graph will fluctuate. It is better to group dates into 3 or 6 months so that there are no large errors.
Now we can see the dynamics of changes in the time of tasks execution for the year, compare the indicators for each type of work.