PGFPlots: How to draw a label at a zero of a function?

Question

Is there an easy method to construct a node (and hence a label) at a zero of a function, i.e. at the intersection of the function with the x-axis? The examples I found were somehow too complicated, and following the KISS principle, I was wondering if there is an elementary method to achieve this.

Edit: To be a bit more precise: I would like to obtain a tick label at the zero of an arbitrary function. The zero should be calculated by the program (so if this is automation, then automation is needed).

Answer 1

Here is an example of how to automatically compute the zero of the function using the intersections library:

Notes:

• I don't know how to extract the x value of the intersection. The solution from Extract x, y coordinate of an arbitrary point in TikZ can extract the coordinate, but that needs to be converted to an axis cs value.

Code

\documentclass[border=2pt]{standalone}
\usepackage{pgfplots}
\usetikzlibrary{intersections}
\begin{document}
\begin{tikzpicture}
    \begin{axis}[
            axis x line=middle,
            axis y line=middle,
            domain=-4:7,
            xmax=7,
        ]
        \addplot[no marks,blue,thick, name path global=My Graph] {xx-4x-7};
        \addplot[no marks,draw=none, name path global=x Axis] {0};
        \fill[red,name intersections={of=My Graph and x Axis,total=\t}]
            \foreach \s in {1,...,\t}{
                (intersection-\s) circle (2pt) 
            };
    \end{axis}
\end{tikzpicture}
\end{document}

Answer 2

Expanding on @Peter Grill 's solution, here is how to calculate the x coordinates.

\documentclass[border=2pt]{standalone}
\usepackage{pgfplots}
\usetikzlibrary{intersections}

\newlength{\len}
\newlength{\plotwidth}
\newcommand{\getvalue}[1]{\pgfkeysvalueof{/pgfplots/#1}}

%output will be given by \pgfmathresult
\newcommand{\xcoord}[1]% #1 = node name
{\pgfplotsextra{%
  \pgfextractx{\len}{\pgfpointdiff{\pgfplotspointaxisxy{0}{0}}{\pgfpointanchor{#1}{center}}}%
  \pgfextractx{\plotwidth}{\pgfpointdiff{\pgfplotspointaxisxy{\getvalue{xmin}}{0}}%
  {\pgfplotspointaxisxy{\getvalue{xmax}}{0}}}%
  \pgfmathparse{\len*(\getvalue{xmax}-\getvalue{xmin})/\plotwidth}%
}}

\begin{document}

\begin{tikzpicture}
    \begin{axis}[
            axis x line=middle,
            axis y line=middle,
            domain=-4:7,
            xmax=7,
        ]
        \addplot[no marks,blue,thick, name path global=My Graph] {x*x-4*x-7};
        \addplot[no marks,draw=none, name path global=x Axis] {0};
        \path[name intersections={of=My Graph and x Axis,total=\t}];

        \draw[very thin,color=gray] (intersection-1) -- +(0,-5mm) coordinate(tick1);
        \xcoord{intersection-1}%
        \node[below] at (tick1) {\pgfmathresult};

        \draw[very thin,color=gray] (intersection-2) -- +(0,-5mm) coordinate(tick2);
        \xcoord{intersection-2}%
        \node[below] at (tick2) {\pgfmathresult};
    \end{axis}
\end{tikzpicture}
\end{document}

Answer 3

Just for typing exercise with PSTricks.

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

\psset{yunit=.5}
\def\f(#1){((#1)^2-4*(#1)-7)}
\begin{document}
\begin{pspicture}[algebraic](-3,-12)(7.5,7.5)
    \psaxes[Dy=2,Dx=2]{->}(0,0)(-3,-12)(7,7)[$x$,0][$y$,90] 
    \psplot[linecolor=blue,linewidth=1pt]{-2}{6}{\f(x)}
        \FPqsolve{\xa}{\xb}{1}{-4}{-7}
        \FPeval\xa{round(xa:2)}\FPeval\xb{round(xb:2)}
    \pstInterFF[PointNameSep=17pt,PosAngle=30,PointName=\xa]{\f(x)}{0}{-2}{A}
    \pstInterFF[PointNameSep=17pt,PosAngle=150,PointName=\xb]{\f(x)}{0}{5}{B}
\end{pspicture}
\end{document}