\usepackage {pstcol}
\begin{pspicture}(0,0)(4,2)\showgrid
 \psline[linearc=0.25,showpoints=true]%
    {->}(1,0)(4,0.3)(2,2)(0,0.5)
  \psline[linestyle=dashed,
    linecolor=blue,
    arrowlength=3]{<->}(0,1)(3,0)(4,2)
\end{pspicture}