Source: https://www.researchgate.net/profile/Mohamed_Mourad_Lafifi/post/How-to-denote-letters-to-mark-significant-differences-in-a-bar-after-pairwise-comparison-in-graphpad-prism/attachment/5fa6da3f7600090001f3b085/AS:955230678167552@1604756149340/download/Prism-6-Statistics-Guide.pdf