This shows you data for the frequency of choosing to upgrade your tavern on different turns. This can be largely dependent on the hero chosen and their playstyle, so you’ll find this chart on each hero’s page.
The solid bars indicate the most common upgrade that can happen on those individual turns. The colors can be matched with the tiers shown below the turn numbers. You’ll also notice the dotted line which shows you how many people are upgrading to that tier on turns where it’s not the most popular move. If you’re interested in just seeing the data on one tavern tier, just hover over one of its solid bars. This will show you only that tavern’s data and give you a better idea of when other players are upgrading to that tier.