Is there a way to add multiple lines of numbers using a "summation line"? Specifically, I want to create something like the below, only better looking.
\documentclass{article}
\usepackage{fontspec,booktabs}
\setmainfont{TeX Gyre Termes}
\begin{document}
\begin{tabular}{ @{} c @{ } c @{ } c @{ } c @{ } c @{ } c @{ } c @{} }
$f(x=1)$ & $=$ & $\sum_{y}f(x=1, y)$ & $=$ & $0,2 + 0,1$ & $=$ & 0,3\\
$f(x=2)$ & $=$ & $\sum_{y}f(x=2, y)$ & $=$ & $0,3 + 0,1$ & $=$ & 0,4\\
$f(x=3)$ & $=$ & $\sum_{y}f(x=3, y)$ & $=$ & $0,1 + 0,2$ & $=$ & 0,3\\
& & & & & $+$ & \\\cmidrule(l){6-7}
& & & & & & 1,0\\
\end{tabular}
\end{document}
I am not interested in doing row or column based mathematical operations. I just want to create a nicer looking summation rule (instead of the one on the lower right part of the table).