dashed line in picture environment

Question

I have drawn the Figure in the picture environment and I have these long lines (which act as the x1-x2 axes) in it. I would like to make these lines dashed, though.

Some googling turned up a command, \dashedline, but my editor (TeXstudio) says that it's an unknown command. Am I using it wrong? Should I add an extra package? I cannot find any information about it all.

Thanks!

Edit: I create the picture using the following code:

\begin{figure}[H]
    \hspace{2.8cm}
    \begin{picture}(300,125)
    \put(-5,70){\vector(-1,0){25}}
    \put(-53,67.5){$-\infty$}   

\put(-5,70){\line(1,0){280}} %x1 axis
\put(266,62){$x_1$}

\put(250,70){\vector(1,0){25}}
\put(278,67.5){$+\infty$}
\put(40,115){\vector(2,-1){90}}

\put(60,97){\line(1,2){6}}
\put(70,92){\line(1,2){6}}
\put(80,87){\line(1,2){6}}


\qbezier(115,78)(122,85)(130,80)
\put(117,83){\footnotesize $\theta$}

\linethickness{0.7 mm}
\put(130,-4){\line(0,1){150}}

\multiput(130,18)(11,0){10}{
    \multiput(0,0)(0,11){10}{+}}
\thinlines

\put(130,-5){\line(0,1){160}} %x2 axis  
\put(132.5,167){$x_2$}

\put(130,147){\vector(0,1){25}}
\put(118.5,175){$+\infty$}      

\put(130,-5){\vector(0,-1){25}}
\put(118.5,-37){$-\infty$}

\end{picture}
\vspace{1cm}
\caption{}
\label{fig:geom}
\end{figure}

Answer 1

Use \multiput to draw many short lines which form a dashed line:

\documentclass{article}

\begin{document}

\begin{figure}
    \hspace{2.8cm}
    \begin{picture}(300,125)

\multiput(-30,70)(20,0){9}{\line(1,0){10}}


%    \put(-5,70){\vector(-1,0){25}}
    \put(-53,67.5){$-\infty$}   

%\put(0,70){\line(1,0){280}} %x1 axis
\put(266,62){$x_1$}

\put(130,70){\vector(1,0){145}}
\put(278,67.5){$+\infty$}
\put(40,115){\vector(2,-1){90}}

\put(60,97){\line(1,2){6}}
\put(70,92){\line(1,2){6}}
\put(80,87){\line(1,2){6}}


\qbezier(115,78)(122,85)(130,80)
\put(117,83){\footnotesize $\theta$}

\linethickness{0.7 mm}
\put(130,-4){\line(0,1){150}}

\multiput(130,18)(11,0){10}{
    \multiput(0,0)(0,11){10}{+}}
\thinlines

\put(130,-5){\line(0,1){160}} %x2 axis  
\put(132.5,167){$x_2$}

\put(130,147){\vector(0,1){25}}
\put(118.5,175){$+\infty$}      

\put(130,-5){\vector(0,-1){25}}
\put(118.5,-37){$-\infty$}

\end{picture}
\vspace{1cm}
\caption{}
\label{fig:geom}
\end{figure}

\end{document}