This visualization depicts specs diversity in the top 2000 Mythic Plus runs for each week. In this context, a week is defined by WoW's weekly reset periods. When a new week starts, the diversity might seem higher because fewer high-level keys have been done yet.
The data is gathered from the World of Warcraft Game Data APIs, specifically the Mythic Keystone Leaderboard API.
The percentage shown beside each spec name represents how often that spec appeared among the top keys across all groups; a value of 100% means it was present in every group. This now applies to all roles, whereas previously it only applied to tanks and healers.