\documentclass{article}
\usepackage{tikz}

\usepackage{pgfplots}
\pgfplotsset{compat=1.5.1}

\begin{document}

%1: latex as calculator
\begin{tikzpicture}
\begin{axis}
[xlabel=$x$,
ylabel=$f(x)$]
	\addplot{x^2+4};

\end{axis}
\end{tikzpicture}

%2: coordinates
\begin{tikzpicture}
\begin{axis}
[xlabel=$x$,
ylabel=$f(x)$]
	\addplot [color=black, mark=o, mark size=12pt]coordinates{
	(0,0)
	(1,2)};
\addlegendentry{Data}
\end{axis}
\end{tikzpicture}

%3: csv
\begin{tikzpicture}
\begin{axis}[
%legend style={draw =none},
legend pos =  north west, % south west, south east, outer
legend columns = 2
]
	\addplot table[x=time, y=voltaje, col sep=comma]{data.csv};
\addlegendentry{voltaje}
	\addplot table[x=time, y=corriente, col sep=comma]{data.csv};
\addlegendentry{Corriente}
\end{axis}
\end{tikzpicture}

\end{document}