Chrysanthemum and Plum is a couple tag used in Hetalia: Axis Powers. It involves the two characters Japan and Taiwan.
For illustrations that contains love elements between the two, it is adviced to tag illustrations together with Normal Couple APH.
The origin of this tag came from both national flowers of their respective countries : Chrysanthemum for Japan, and Plum for Taiwan.
An early illustration by Hidekaz Himaruya has her poking China on the cheek and telling him to leave Japan alone, which indicates that she prefers Japan over China and has led some fans to believe that she has a crush on Japan.
Due to their lack of canon interaction, fan interpretations of Japan and Taiwan's relationship are largely speculative, but it's generally assumed that China raised the other Asian nations as a family unit and that Japan and Taiwan hence likely grew up together, which would have given them time to interact with and potentially develop feelings for each other.
Hetalia: Axis Powers
Three Person Group
Rotten 3 Families
Chrysantemum Plum Orchid
Asia Sun Trio
KikuWan -> Japan×Taiwan