# 3D plot with pgfplots

I am trying to plot the function $U(x,y)=6*\ln(x) + \ln(y)$ for $x=1,...,4$ and $y=1,...,14-3*x$, however, I can't specify that the domain for $y$ is a function of $x$.

MWE:

\documentclass[tikz,border=5pt]{standalone}
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}
\begin{axis}[view/h=40,colormap/violet]
surf,
samples=50,
domain=1:4,y domain=1:14-3*x] % it goes wrong here
{6*ln(x) + ln(y)};
\end{axis}
\end{tikzpicture}
\end{document}


How can I do this?

UPDATE:

I have tried to use \pgfmathparse and \pgfmathresult, but I still can't get it to display the right surface.

MWE:

\documentclass[tikz,border=5pt]{standalone}
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}
\begin{axis}[
%view/h=80,
colormap/cool,
xlabel = $x$,
ylabel = $y$,
zlabel = {$U(x,y)$}
]

\foreach \i in {1,...,4}{
\pgfmathparse{14 - 3*\i};
surf,
samples=50,
domain=1:4,y domain=1:\pgfmathresult]
{6*ln(x) + ln(y)};
}
\end{axis}
\end{tikzpicture}
\end{document}

• – Bobyandbob Oct 8 '17 at 6:45
• @Bobyandbob I tried to follow the last approach, but I couldn't get it to work. I have updated my question with the new code. – lala_12 Oct 8 '17 at 7:04

One approach could be using:

\addplot3[
surf,
samples=60,
domain=1:4,
y domain=1:14]
{y < 14-3*x ? 6*ln(x) + ln(y) : nan};
\end{axis}


which seems to almost work, because I have an error (the same as here) and I cannot see why. If you change nan (not-a-number) with 0, there is no error. Probably the shader is confused by the big number of NaN arounds...

As suggested by the OP, adding

restrict z to domain=-inf:inf,


to the axis options does the trick:

(although the ragged end is not so nice...)

• Adding the line restrict z to domain=-inf:inf to \begin{axis} does the trick. Thank you. – lala_12 Oct 8 '17 at 20:56