The term for the negative equivalent of a peak point on a graph

You're looking for "nadir"

• nadir - (noun) - the worst or lowest point of something.

Examples abound :

• "That period was the nadir of his presidency."

• "The stock exchange reached a nadir today."

• "By 1600 Apalachee had experienced a 17 percent reduction in population size and reached a nadir of 73 percent by the first decade of the 18th century.

• "Public support for the LDP reached a nadir in the 1970s.

• "Thus population growth was quite slow in the 19th century, and the nadir was reached in the first half of the 20th century when France"

• " With the rise of Napolean, Spanish fortunes reached their nadir.

Mathematically speaking, it is called a global/absolute minimum. The smallest value in a set of minima.