\documentclass{article}
\usepackage{tikz}
\usepackage{pgfplots}
\pgfplotsset{compat=1.5.1}

\begin{document}
%1 latex as calculator
\begin{tikzpicture}
\begin{axis}[
	xlabel=tiempo $t$,
	ylabel=$f(t)$, mark repeat=2]
	\addplot{x^2+5};
	\addplot{x+6};
\end{axis}

\end{tikzpicture}

%2 coordinates
\begin{tikzpicture}
\begin{axis}[
	xlabel=$x$,
	ylabel=$f(x)$]
	\addplot[color=black, mark=diamond, mark size=4pt] coordinates{
	(0,0)
	(1,4)
	(2,9)
	(3,-3)
	};
	\addlegendentry{pH}
\end{axis}

\end{tikzpicture}

%3 CSV files:
\begin{tikzpicture}
\begin{semilogyaxis}[
	legend pos=north west,
	%ylabel style={rotate =-90},
	xlabel=timpo en ($s$),
	ylabel=Voltaje ($mV$)]
	\addplot[color=black,dashed, mark=o, line width=3pt] table[x=tiempo, y=voltaje, col sep=comma]{data.csv};
	\addlegendentry{voltaje}
	\addplot table[x=tiempo, y=corriente, col sep=comma]{data.csv};
		\addlegendentry{corriente}
\end{semilogyaxis}

\end{tikzpicture}



























\end{document}