Inserting and correctly scaling PGF figure made with matplotlib

Question

I created a plot in matplotlib and saved it as a PGF file.

I insert the figure into Latex with the following:

\begin{figure}
    \centering
    \input{test_fig.pgf}
\end{figure}

With \usepackage{pgf} in the preamble.

The figure is then inserted but its to big, i.e. stretching far off the page.

I should mention that this is in a beamer document class.

Is there a method to scale the figure so that its as wide as the \textwidth?

I have tried inserting the figure with \includegraphics[width=\textwidth]{test_fig.pgf}, but I get the error Unknown graphics extension: .pgf.

Here is a MWE, with the PGF figure attached here:

\documentclass{beamer}
\usepackage{beamerthemeshadow}
\usepackage{pgf}

\begin{document}
\title{Slide Title}  
\author{Author}
\date{\today}
\frame{\titlepage} 
\section{Section 1} 
\frame{\frametitle{Title}
\begin{figure}
    \centering
    \resizebox{0.8\textwidth}{!}{\input{test_fig.pgf}}
\end{figure}
}
\end{document}