We improved the Review Collaboration report, by centralizing the data and trendline reports from the Submitter and Reviewer Fundamentals reports into a single report - Review Collaboration. 

And by hovering your cursor over contributor profile pictures, you can now see more data about collaboration dynamics in your teams, such as:

• who reviews whose PRs 
• whose PRs are being reviewed by whom
• who reviewed the most/ least PRs
• who submitted the most/ least PRs