I'd like to link two points smoothly. I tried to do it with the following code:

\begin{document}

\begin{center}
\begin{tikzpicture}
\draw (0.7,0) -- (0.7,6);
\draw (0,6) -- (0,-0.8);
\draw (0.5,-1.2) -- (2.2,-1.2);
\draw plot [smooth] coordinates {(0,-0.8) (0.5,-1.2)};
\end{tikzpicture}
\end{center}
\end{document}


Thank you very much in advance.

Option rounded corners can help, e.g.:

\documentclass{article}
\usepackage{tikz}
\begin{document}
\begin{center}
\begin{tikzpicture}
\draw[rounded corners=10pt] (0, 1) -- (0, -1.2) -- (1.2, -1.2);
\end{tikzpicture}
\end{center}
\end{document}


The smoothest curve joining two points is straight.

You want a Bézier curve:

\documentclass[border=4,tikz]{standalone}

\begin{document}

\begin{tikzpicture}
\draw (0,1) -- (0,-0.8) .. controls (0,-1.2) .. (0.5,-1.2) -- (1.2,-1.2);
\end{tikzpicture}

\end{document}