RC filter with an square wave input (checking my math and find my error, if it is somewhere)
As a reality check, just to make sure the initial part of the response is correct:

The initial input is a step of height, A and the response to this is, therefore, \$\small A\frac{RC}{1+sRC}=A\frac{1}{s+1/RC}\$

This inverse transforms to \$\small Ae^{-t/RC}\$, which appears to indicate \$\small A=-6\$. I suspect the remainder of the response shown is correct, but the graph may be incomplete (top half missing?).