Why does anger has something to do with spleen in both Chinese and English?
It is my speculation


  https://baike.baidu.com/item/%E8%84%BE/20358750
  
  中医解读:
  
  脾气的运动特点是主升举。
  
  脾为太阴湿土，又主运化水液，故喜燥恶湿。


Translation:


  Interpretation of Chinese medicine:
  
  The typical movement of the air in the spleen is upward rising 
  
  Spleen is a cool, wet, water filtering organ. it likes dry and hot and dislikes wet and cool


Anger is also associate with 燥(dry) and hot . We say a person is easy to anger when his being 燥.

My assumption is: A person being angry because his 脾 is overly active causing 燥 (dryness) inside one's body and dryness cause anger

Related phrase:


發脾气 (being angry)
大脾气 (hot temper)
脾气好 (mild temper)
脾气差 (bad temper)