How does Matlab calculate the fit line in the regression graph?

I am using right now the neural network toolbox for my problem. The regression graph, which is calculated and represented by Matlab, is unclear. When you have multi-regression problem, which means a vector for each sample at the input and output, then it's difficult to plot such fit line for the samples. How could Matlab do this?

https://de.mathworks.com/help/deeplearning/gs/fit-data-with-a-neural-network.html

Could somebody help?