summation line for multiple rows

Question

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).

Answer 1

I am used to the plus being to the right of the last line being summed. This can be acheived with:

\documentclass{article}
\usepackage{booktabs}

\begin{document}

\begin{tabular}{@{}c@{ }c@{ }c@{ }c@{ }c@{ }c@{ }c@{\hspace{0.5em}} c}
$f(1)$ & $=$ & $\sum_{y}f(1, y)$ & $=$ & $0{,}2 + 0{,}1$ & $=$ & 0{,}3 &         \\
$f(2)$ & $=$ & $\sum_{y}f(2, y)$ & $=$ & $0{,}3 + 0{,}1$ & $=$ & 0{,}4 &         \\
$f(3)$ & $=$ & $\sum_{y}f(3, y)$ & $=$ & $0{,}1 + 0{,}2$ & $=$ & 0{,}3 & \( + \) \\
\cmidrule(r){7-8}
       &     &                   &     &             &     & 1{,}0 &         \\
\end{tabular}

\end{document}

adding an extra column to hold the plus sign. I have also written {,} for better spacing on your decimal separator, and removed the x=.