[Tex/LaTex] How to change the aspect ratio of gnuplot output

Question

How do I change the aspect ratio of the canvas of a gnuplot plot in LaTeX using the gnuplottex package?

The following changes the aspect ratio of the output but not the canvas size, thus leaving a margin above and below the graph.

\documentclass{scrreprt}

\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{lmodern}
\usepackage[miktex,cleanup]{gnuplottex}   %miktex option for my editor
\usepackage{graphicx}
\usepackage{epstopdf}

\begin{document}

\begin{figure}[htbp]
    \centering
    \begin{gnuplot}[terminal=cairolatex]
    set size ratio 0.2
    plot [0:2*pi] sin(x) with lines linewidth 2 title 'Sine'
    \end{gnuplot}
\end{figure}

\end{document}

Answer 1

I'm not fluent in gnuplot, so this could be refined. You can pass parameters based on \textwidth, as long as they are in the options to gnuplot:

\documentclass{scrreprt}

\usepackage{lipsum}
\usepackage[cleanup]{gnuplottex}
\usepackage{xparse}
\ExplSyntaxOn
\DeclareExpandableDocumentCommand{\convertlen}{ O{cm} m }
 {
  \dim_to_decimal_in_unit:nn { #2 } { 1 #1 } cm
 }
\ExplSyntaxOff

\begin{document}

\lipsum[2]

\begin{figure}[htbp]
    \centering
    \begin{gnuplot}[
      terminal=cairolatex,
      terminaloptions={size \convertlen{\textwidth},\convertlen{.3\textwidth}}
    ]
    set size ratio 0.3
    plot [0:2*pi] sin(x) with lines linewidth 2 title 'Sine'
    \end{gnuplot}
\end{figure}

\lipsum[2]

\end{document}

The \convertlen macro is just to provide gnuplot with something it knows (centimeters, in this case). The command has also an optional argument for specifying another unit.