4

I'm trying to draw the graph of a curve in three dimensions. For some reason, tikz/pgfplots is automatically closing the curve. I can't figure out how to stop it from doing this. Here is an example:

\begin{tikzpicture}
  \begin{axis}
    \addplot3 [
      variable=t,
      domain=-1.5:1.5] ({t},{0},{20-t^2});
  \end{axis}
\end{tikzpicture}

What it produces for me is the following:

enter image description here

For some reason, it has connected the start and end points of the curve.

1
  • For reference, this is described in the pgfplots manual, in the section about \addplot3. In the manual for version 1.18.1, it's section 4.6.2 The \addplot3 Command: Three Dimensional Coordinate Input, on page 125. Quote: "The “auto-completion” depends on how you provide your data. • If you use \addplot3 expression, mesh/rows and mesh/cols are computed from the values samples and samples y. By default, \addplot3 expression always samples a mesh. If you want it to sample a line, set samples y=1 (or, equivalently, y domain=0:0)." Jun 28 at 15:29
7

You need to tell PGFPlots to only draw the curve once with samples y=1:

\documentclass[tikz]{standalone}
\usepackage{pgfplots}
\begin{document}\begin{tikzpicture}
  \begin{axis}
    \addplot3 [variable=t, domain=-1.5:1.5, samples y=1] ({t},{0},{20-t^2});
  \end{axis}
\end{tikzpicture}   
\end{document}

Graph

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