\setlength{\unitlength}{0.8cm}
\begin{picture}(8,9)
\thicklines
\put(2,1){\vector(1,0){7}}
\put(2,1){\vector(0,1){7}}
\put(1.5,8){$t$}
\put(9.3,.8){$x$}
%\thinlines
\put(0,5){\line(1,-1){7}}
%\put(0,5){\line(1,0){5}}
\put(2,2){\circle*{2}}
\end{picture}