how to draw a negative exponential plot of this situation with python
The model you are looking for is this:

$Y=A e^{-Bx} + c$, I know the implementation in R, because I don't know of a nonlinear estimation in Python

This code in R might work:

R=data.frame(X=c(1,2,3,4,5,6,7,8,9),Y=c(1,2,3,3,3,3,3,3,3)) # Data in which X, Y are your data
model=nls(formula = Y~A*exp(-B*X)+C,data=R)
summary(model)


There is a limitation to take into account, is explained in this link, is summarized in the impossibility for all possible models to exist, the "most inside" model should be linear.

First steps with Non-Linear Regression in R

Singular Gradient Error in nls with correct starting values