Example of two overlapped plots in LaTeX/Tikz.

plotTwoFunctions.tex

\documentclass{article}

\usepackage{tikz}
\usepackage{pgfplots}

\begin{document}

    \begin{tikzpicture}
     \begin{axis}[
          xlabel=$x$,
          ylabel=$y$,
          domain=-2:4
     ]
     \addplot[color=red] {sin(deg(x))};
     \addplot[color=blue] {cos(deg(x)};
     \legend{$\sin(x)$,$\cos(x)$}
     \end{axis}

\end{tikzpicture}
\end{document}