Add PTO information to Development Department Narrow MR Rate chart

It is nice to have visibility of correlation between the Narrow MR Rate and PTO Time in one place. Can we display the data in a single chart somehow?

Because PTO data is only available at Development Department level at the moment, the first iteration can start from the department level. When the PTO data is broken down to sub-department and group levels (tracking issue https://gitlab.com/gitlab-com/team-member-epics/access-requests/-/issues/6464), it's very desirable to make the correlation visible in sub-departments and groups.