The above function looks like this:

enter image description here

However, using tikzpicture with addplot

\documentclass[11pt]{article}
\usepackage{pgfplots}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
  \begin{axis}
    \addplot[domain=-6:6,samples=50,smooth,red] { (sin(x)^2)/pow(x,2) };
  \end{axis}
\end{tikzpicture}
\end{document}

Gives the following disturbing result:

enter image description here

How should I plot sin(x)^2 / x^2 in tikz?

  • 3
    (1) welcome, (2) as always on this site you are much much more likely to get help if you provide a full minimal example that others can copy and test as is. Then they will not have to guess 80% of your setup. – daleif Nov 20 at 10:53
  • 1
    Thanks for the tip, I've done that now! – MrMartin Nov 20 at 10:57
  • 8
    Try more samples and sin(deg(x)) – daleif Nov 20 at 10:58
  • Yes, it seems sin(deg(x)) is required when plotting sin(x). Thanks! – MrMartin Nov 20 at 11:23
  • 1
    @daleif Damn! I did not see your comment :) Also sampling is just as fine, as it is now. Only deg(...) is necessary with sin. – Raaja Nov 20 at 11:41
up vote 8 down vote accepted

As commented by @daleif, the problem is that the trigonometric functions in pgfplots are defined with the argument in degrees, not in radians. So you are calculating the function really near zero, and you have huge rounding errors (and wrong scale...).

You can use sin(deg(x)) to do the conversion, or globally switch with \pgfplotsset{trig format=rad}.

Notice however that the manual states:

/pgf/trig format=deg|red [sic, should be rad]

Allows to reconfigure the trigonometric format for all user arguments. This affects all user arguments including view, Tik Z polar coordinates, pins of \nodes, start/end angles for edges, etc. At the time of this writing, this feature is in experimental state: it can happen that it breaks Tik Z internals. Please handle with care and report any bugs.

In this case it works:

\documentclass[margin=10pt]{standalone}
\usepackage{pgfplots}\pgfplotsset{compat=newest}
\usepackage{tikz}
\begin{document}
\pgfplotsset{trig format=rad}
\begin{tikzpicture}[]
  \begin{axis}
    \addplot[domain=-6:6,samples=50,smooth,red] { (sin(x)^2)/pow(x,2) };
  \end{axis}
\end{tikzpicture}
\end{document}

enter image description here

Notice also that you have to avoid the point x=0 in your drawing, where the function is undefined... this is with:

\addplot[domain=-6:6,samples=151,smooth,red] { (sin(x)^2)/pow(x,2) };

enter image description here

A PSTricks solution for comparison purpose only.

\documentclass[pstricks,border=12pt]{standalone}
\usepackage{pst-plot}

\def\f{(sin(x)/x)^2}
\psset
{
    yunit=4cm,
    xunit=.5cm  
}   

\begin{document}
\begin{pspicture}[algebraic,plotpoints=200](-12,-.1)(12.5,1.2)
    \psaxes[Dx=5]{->}(0,0)(-12,-.1)(12,1.1)[$x$,0][$y$,90]
    \psplot[linecolor=blue]{-12}{12}{\f}
    \uput[45](*.7 {\f}){$\displaystyle f(x)=\frac{\sin^2(x)}{x^2}$}
\end{pspicture}
\end{document}

enter image description here

It seems that the problem is with pgfplots, and it can be solved by switching to gnuplot:

\documentclass[11pt]{article}
\usepackage{pgfplots}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
  \begin{axis}
    \addplot [no markers] gnuplot [domain=-6:6, samples=50] {sin(x)**2/x**2};
  \end{axis}
\end{tikzpicture}
\end{document}
  • 1
    Nope, works just fine for me without gnuplot. – daleif Nov 20 at 11:17

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