# Draw a point with a label in a defined function PGF Plots

i would like to plot a function with PGF and also plot some points that belong to a function with a label. I defined the function and plotted it with this code:

\begin{tikzpicture} [declare function = {f(\x) = -0.05*\x^3 -0.5*\x^2;}]
\begin{axis}
\addplot [blue, domain=-10:10] {f(x)};

\end{axis}
\end{tikzpicture}


And i tried to mark the points with labels with \node, but i always get errors when i try to do so letting PGF calculate the y-value using the declared function f(\x). How can i solve my problem?

• Welcome to TeX.SX! Please make your code compilable (if possible), or at least complete it with \documentclass{...}, the required \usepackage's, \begin{document}, and \end{document}. That may seem tedious to you, but think of the extra work it represents for TeX.SX users willing to give you a hand. Help them help you: remove that one hurdle between you and a solution to your problem. – Rmano Feb 18 at 18:25

## 2 Answers

Next time, please post a complete example --- a lot depends on your preamble. Anyway, I think that you are looking for this:

\documentclass[border=10pt]{standalone}
\usepackage{tikz}
\usepackage{pgfplots}\pgfplotsset{compat=1.16}
\usetikzlibrary{arrows.meta,positioning,calc}
\begin{document}
\begin{tikzpicture} [declare function = {f(\x) = -0.05*\x^3 -0.5*\x^2;}]
\begin{axis}
\addplot [blue, domain=-10:10] {f(x)};
\path (-1,{f(-1)}) node[circle, fill=red]{}
node[below=4pt ]{-1, \pgfmathparse{f(-1)}\pgfmathresult};
\end{axis}
\end{tikzpicture}
\end{document}


In coordinates, you have to hide the () to the parser, so you have to protect them in a couple of braces. • Thank you! Solved my problem! – Andrew Feb 19 at 12:44

You weren't very specific so I present only two solutions to your problem. The advantage over Rmano's answer is, that it works for any \addplot command, i.e. not only for the ones that plot a given function.

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

% used PGFPlots v1.17
\documentclass[border=5pt]{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=1.12}
\begin{document}
\begin{tikzpicture}[
declare function={
f(\x) = -0.05*\x^3 - 0.5*\x^2;
},
]
\begin{axis}
% first plot the function
\addplot+ [
domain=-10:10,
% here we only want the line
no markers,
% in case the next plot should use the same color
% (from the cycle list)
forget plot,
] {f(x)};

% second set the marks at the desired positions
\addplot+ [
% here we only want the markers ...
only marks,
% ... at the positions in this list ...
samples at={-9,-5},
% ... together with their corresponding y value
% (which is the default) ...
nodes near coords,
] {f(x)};

\addplot+ [
only marks,
samples at={-1,3},
% ... but you can define it your needs, e.g.
nodes near coords={%
\footnotesize
$(\pgfmathprintnumber{\pgfkeysvalueof{/data point/x}}, \pgfmathprintnumber{\pgfkeysvalueof{/data point/y}})$%
},
% but you dan do a lot more stuff.
% If you are curious, search in the PGFPlots manual for
% "nodes near coords" or here on TeX.SX for the tag "nodes near coords"
% (https://tex.stackexchange.com/questions/tagged/nodes-near-coords)
] {f(x)};
\end{axis}
\end{tikzpicture}
\end{document} 