Q:

Pgfplots with custom axis markers

Assume you have a trigonometric function, say for simplicity sin(x). When plotting the function, how can one place the x-axis markers on 0, pi/2, pi, 3pi/2, 2pi, instead of the default numeric values that pgfplots uses?

A:

Use \xtick to specify where you want the ticks, and \xticklabels to specify the corresponding labels. 
\documentclass{article} 
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}
\begin{axis}[
    axis x line=center, 
    axis y line=middle, 
    xtick={0,1.5708,3.14159,4.7123889},
    xticklabels={$0$,$\frac{\pi}{2}$,$\pi$,$\frac{3\pi}{2}$},
    domain=0:2*pi
]
\addplot {sin(deg(x))};
\end{axis}
\end{tikzpicture}
\end{document}

If you don't want a label a particular tick mark, just leave that one blank. For example, if you did not want the label at pi you would replace the line with xticklabels={$0$,$\frac{\pi}{2}$,,$\frac{3\pi}{2}$}.

A:

You can specify the position of the ticks using the xtick and scaled x ticks options. Example:
\documentclass{article} 
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}
\begin{axis}[xtick={0,1.5708,...,10},domain=0:2*pi,scaled x ticks={real:3.1415},
  xtick scale label code/.code={$\cdot \pi$}]
\addplot {sin(deg(x))};
\end{axis}
\end{tikzpicture}
\end{document}

Taken from page 183 of the manual.

A:

Using gnuplot, I suggest the following:
\documentclass{minimal}
\usepackage{tikz}
% GNUPLOT required
\begin{document}
\begin{center}
  \begin{tikzpicture}[domain=0:6.3]
    \foreach \x/\xtext in {0/0, 0.5*pi/\frac{\pi}{2} ,pi/\pi,
      1.5*pi/\frac{3\pi}{2}, 2*pi/2\pi}
    \draw[shift={(\x,0)}] (0pt,2pt) -- (0pt,-2pt) node[below] {$\xtext$};
    \draw[very thin,color=gray] (-0.1,-1.1) grid (2*pi,1.1); 
    \draw[color=blue] plot[id=sin] function{sin(x)} node[right] {$f(x) = \sin x$};
  \end{tikzpicture}
\end{center}
\end{document}

Which is maybe simpler. Maybe not... I'm using explicit representation of the axis' labels.

