%documentclass[tikz]{standalone}
documentclass{beamer}
usepackage{tikz}
usepackage{pgfplots}
begin{document}

begin{frame}

begin{tikzpicture}
begin{axis}
addplot[blue, thick] {x};
only<2>{addplot[blue, thick, dashed] {2*x};}
end{axis}
end{tikzpicture}
end{frame}

end{document}