Avg sprint still-open share(2.4.2)

The average of each sprint's share of issues still open now, over every sprint Sprint Scope Still Open reads for the selected project and sprint filters (the date range does not apply), including sprints older than the 40 the chart draws, where each issue counts in the sprint it was most recently assigned to and issues deleted in Jira, in a trashed or deleted project, or whose events record no status that maps to a Jira status category are left out. Each sprint weighs the same in the average whatever its size, so read it beside the per-sprint bars rather than as one sprint's figure.