# Coordinates of intersections

In axis environment I have a curve and a line intersecting it. How can I get components of (intersection-1), (intersection-2) etc. in axis units?

\documentclass{minimal}
\usepackage{pgfplots}
\usetikzlibrary{intersections}

\begin{document}

\begin{tikzpicture}
\begin{axis}
\path[name path global=HelperLine] (axis cs:48,0.13288) -- (axis cs:56,0.13288);
\draw[dashed,name intersections={of=GaussCurve and HelperLine}] (axis cs:48,0.13288) -- (intersection-2);
\fill[red] (intersection-2) circle (.1cm);
\end{axis}
\end{tikzpicture}
\end{document}


e.g. how to get the x component of red circle (intersection-2)?

This code generated by program from some data, so I can't use the presented values, it's just example.

• Please provide a minimum working example, a smallest possible compilable example that shows the exact problem. Like this, it's easier for others to help you. – gerrit Dec 16 '11 at 9:36
• You can use \pgfgetlastxy{\macrox}{\macroy}. This will set \macrox and \macroy to the x and y component of the last used coordinate respectively. So \path (intersection-2); \pgfgetlastxy{\macrox}{\macroy} will set \macrox to the x coordinate of (intersection-2). – Roelof Spijker Dec 16 '11 at 10:03

As wh1t3 said in the comment, you can extract the coordinate using \pgfgetlastxy{<macro for x>}{<macro for y>}. In order to transform this into axis units, you have to apply the inverse of the coordinate transformation that PGFplots uses. In the example below, I've wrapped the transformation in a macro \transformxdimension, which takes a length in pt and sets \pgfmathresult to contain the length in axis units: \documentclass{minimal}
\usepackage{pgfplots}
\usetikzlibrary{intersections}

\begin{document}

\makeatletter
\newcommand\transformxdimension{
\pgfmathparse{((#1/\pgfplots@x@veclength)+\pgfplots@data@scale@trafo@SHIFT@x)/10^\pgfplots@data@scale@trafo@EXPONENT@x}
}
\newcommand\transformydimension{
\pgfmathparse{((#1/\pgfplots@y@veclength)+\pgfplots@data@scale@trafo@SHIFT@y)/10^\pgfplots@data@scale@trafo@EXPONENT@y}
}
\makeatother

\begin{tikzpicture}
\begin{axis}[yticklabel style={/pgf/number format/.cd, fixed, fixed zerofill}]
\path[name path global=HelperLine] (axis cs:48,0.13288) -- (axis cs:56,0.13288);

\draw[dashed,name intersections={of=GaussCurve and HelperLine}] (axis cs:48,0.13288) -- (intersection-2)
node [anchor=south, fill=white, fill opacity=0.75,text opacity=1]{
\pgfgetlastxy{\macrox}{\macroy}
\transformxdimension{\macrox}
\pgfmathprintnumber{\pgfmathresult},%
\transformydimension{\macroy}%
\pgfmathprintnumber{\pgfmathresult}
}
;
\fill[red] (intersection-2) circle (.1cm);
\end{axis}
\end{tikzpicture}
\end{document}

• Fine answer, I can update my code but is it possible with pgfplots to use another tool that node or pgfextra? – Alain Matthes Dec 17 '11 at 17:09
• @Altermundus: You should enclose all your \draw, \path, \pgfgetlastxy, etc., in \pgfplotsextra{...}, which will execute all the commands in the same scope. That way, you don't need to \global\let the result, and you don't have to smuggle the \pgfgetlastxy into a \draw command using \pgfextra. Instead, you can just say \pgfplotsextra{ \draw [name intersections=...] ... ; \pgfgetlastxy{...} }. – Jake Dec 19 '11 at 6:27

I try the idea of whlt3 but it's was not easy; see the next code (perhaps I do some wrong things because I don't know very well pgfplots). I try also \pgfextractx. I need in each case to use \pgfextra to get the x component.

Update with the excellent answer of Jake :

  \documentclass{minimal}
\usepackage{tikz,pgfplots}
\usetikzlibrary{intersections}

\makeatletter
\newcommand\transformxdimension{
\pgfmathparse{((#1/\pgfplots@x@veclength)+\pgfplots@data@scale@trafo@SHIFT@x)/%
10^\pgfplots@data@scale@trafo@EXPONENT@x}
}
\newcommand\transformydimension{
\pgfmathparse{((#1/\pgfplots@y@veclength)+\pgfplots@data@scale@trafo@SHIFT@y)/%
10^\pgfplots@data@scale@trafo@EXPONENT@y}
}
\makeatother

\begin{document}

\begin{tikzpicture}
\begin{axis}
exp(-0.5*((x-52.64)/1.82)**2)/(sqrt(2*pi)*1.82)};
\path[name path global=HelperLine] (axis cs:48,0.13288) -- (axis cs:56,0.13288);
\draw[dashed,name intersections={of=GaussCurve and HelperLine,name=i}] (axis %
cs:48,0.13288) -- (i-2)%
\pgfextra{\pgfgetlastxy{\macrox}{\macroy}%
\global\let\macrox\macrox};
\fill[red] (i-2)  circle (.1cm);
\draw[dashed] (i-2)--(\macrox,0) node {%
\transformxdimension{\macrox}
\pgfmathprintnumber{\pgfmathresult}};
\end{axis}

\end{tikzpicture}

\end{document} • I don't know to transform the result with axis units – Alain Matthes Dec 16 '11 at 13:40

With the release of PGFPlots v1.16 it is now possible to store (axis) coordinates with \pgfplotspointgetcoordinates in data point, which then can be called by \pgfkeysvalueof.

For details please have a look at the comments in the code.

% used PGFPlots v1.16
\documentclass[border=5pt]{standalone}
\usepackage{pgfplots}
\usetikzlibrary{intersections}
\pgfplotsset{
% use this compat' level or higher, so TikZ coordinates don't have to
% be prefixed by axis cs:'
compat=1.11,
}
\begin{document}
\begin{tikzpicture}
\begin{axis}[
yticklabel style={
/pgf/number format/fixed,
},
]
gnuplot [domain=48.00:56.00,samples=100]
{exp(-0.5*((x-52.64)/1.82)**2)/(sqrt(2*pi)*1.82)};
\path [name path global=HelperLine]
(48,0.13288) -- (56,0.13288)
coordinate [at start]   (start)
;
\draw [dashed,name intersections={of=GaussCurve and HelperLine}]
(start) -- (intersection-2) -- (intersection-2 |- 0,0)
% -------------------------------------------------------------
% using \pgfplotspointgetcoordinates' stores the (axis)
% coordinates of e.g. the coordinate (intersection-2) in
% data point', which then can be called by \pgfkeysvalueof'
node [at start,below left] {
\pgfplotspointgetcoordinates{(intersection-2)}
$( \pgfmathprintnumber[fixed]{\pgfkeysvalueof{/data point/x}}, \pgfmathprintnumber[fixed]{\pgfkeysvalueof{/data point/y}} )$
}
% -------------------------------------------------------------
;
\fill [red] (intersection-2) circle (1mm);
\end{axis}
\end{tikzpicture}
\end{document}
` 