Triangular Plot

A triangular plot has three variables that sum up to a constant. It depicts rations of the three variables in an equilateral triangle. This type of map happens to analyze the different soil types, with the three variables being clay, silt, and sand. 

http://www.westone.wa.gov.au/toolbox6/hort6/html/resources/depot/hort_file/pyramid/text_alt.htm