[Tex/LaTex] Plotting the sign function

pgfplotsplot

I'd like to create a graph of the sign function in my document:

using pdfplots and tizkpicture (as used here). Could you please let me know how to define the function in the addplot part?

\documentclass{standalone}
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}
  \begin{axis}[ 
    xlabel=$x$,
    ylabel={$\sgn(x)$}
  ] 
    \addplot {?????}; 
  \end{axis}
\end{tikzpicture}
\end{document}

Best Answer

The following example solves the function drawing by dividing it into parts:

Line at y=-1 from left to the y axis at x=0.
Line at y=1 from the y axis at x=0 to the right.
Red dot at point (0, 0).
White dot with black circle to exclude points at (0, -1) and (-1, 0).

The minus sign of y tick "-1" would be overprinted by the red line. Therefore, the tick is set as extra tick with a style that moves the tick mark to the right.

\documentclass{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=newest}
\usepackage{amsmath}
\DeclareMathOperator{\sgn}{sgn}

\begin{document}
\begin{tikzpicture}
  \begin{axis}[
    axis lines=middle,
    xlabel=$x$,
    ylabel={$\sgn(x)$},
    xmin=-3, xmax=3,
    ymin=-1.5, ymax=1.5,
    xtick=\empty,
    ytick={0, 1},
    extra y ticks={-1},
    extra y tick style={
      tick label style={anchor=west, xshift=3pt},
    },
    function line/.style={
      red,
      thick,
      samples=2,
    },
    single dot/.style={
      red,
      mark=*,
    },
    empty point/.style={
      only marks,
      mark=*,
      mark options={fill=white, draw=black},
    },
  ]
    \addplot[function line, domain=\pgfkeysvalueof{/pgfplots/xmin}:0] {-1};
    \addplot[function line, domain=0:\pgfkeysvalueof{/pgfplots/xmax}] {1};
    \addplot[single dot] coordinates {(0, 0)};
    \addplot[empty point] coordinates {(0, -1) (0, 1)};
  \end{axis}
\end{tikzpicture}
\end{document}

Best Answer

Related Question