Added variable plot for whole model: x-axis meaning

I would like to extend the asnwer question about the adjusted variable plot (bellow), obtained by using the command plotAdded(mdl). It should show the intercept vs. the adjusted plot. What is the meaning of the x-axes. If I choose the point on the curve x=0, then y=0? What if the meaning of e.g. x=-4? Was the plot obtain by applying the Frisch-Waugh-Lovell theorem until only the constant is remaining?

Knowing: "You can interpret that graph as showing your model has explanatory power, if it did not, it would appear as a horizontal line in the middle of the graph"

From the Matlab Answer: Added Variable Plot: Adjusted whole model x-axis range

Thank you very much.