tikz: construct a phantom path

Question

How can I construct a path (curve, line) but not draw it?

For example, I want to construct the following path

\draw[name path = phantom] plot coordinates {
 (0, 2)
 (2, 5)
 (3, 7)
} ;

but do not want the actual line shown.

Answer 1

The command \draw actually is a shortcut for \path[draw], if you remove the draw option, and write only

\path

you'll have a path that is not drawn, even though TikZ will take into account its dimensions.

Possible alternatives are:

\draw[draw=none]

(which is exactly equal to a \path command) where you say to draw a path without any color, or

\draw[opacity=0]

where you say to draw a transparent path. For a detailed explanation of the difference, see: Difference between draw=none and draw opacity=0 (also for fill=none and fill opacity=0)?

Here is an MWE with the various options, I've put a frame around the tikzpictures just to show that, even if the paths aren't drawn, their dimensions are taken into account.

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{intersections}
\usepackage{framed}

\begin{document}
    This is your command:

    \begin{framed}
        \begin{tikzpicture}
            \draw[name path = phantom] plot coordinates {
                (0, 2)
                (2, 5)
                (3, 7)
            }; 
        \end{tikzpicture}
    \end{framed}

    This is with \texttt{\textbackslash path} only:

    \begin{framed}
        \begin{tikzpicture}
            \path plot coordinates {
                (0, 2)
                (2, 5)
                (3, 7)
            } ;
        \end{tikzpicture}
    \end{framed}
    \newpage
    This is with \texttt{\textbackslash draw[draw=none]}:

    \begin{framed}
        \begin{tikzpicture}
            \draw[draw=none] plot coordinates {
            (0, 2)
            (2, 5)
            (3, 7)
            } ;
        \end{tikzpicture}
    \end{framed}

    This is with \texttt{\textbackslash draw[opacity=0]}:
    \begin{framed}
        \begin{tikzpicture}
            \draw[opacity=0] plot coordinates {
                (0, 2)
                (2, 5)
                (3, 7)
            } ;
        \end{tikzpicture}
    \end{framed}
\end{document}

Answer 2

An extension to the list of CarLaTeX.

Discard layer

The source code TDS:tex/generic/pgf/basiclayer/pgfcorelayers.code.tex reveals a special layer that is discarded (it is not documented in the TikZ manual 3.0.1a):

\def\pgfdiscardlayername{discard}

\def\pgfonlayer@assert@is@active{%
  \ifx\pgfonlayer@name\pgfdiscardlayername
     % this special layer name can be used as /dev/null without
   % warning.
...

The next file intersects the plot of the question with a circle, adds a node in the center and draws a line from the intersection points to the left and right of the node:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{intersections}

\begin{document}
\begin{tikzpicture}
  \path (2, 5) node[draw] (hw) {Hello World};
  \draw[name path=phantom] plot coordinates {
    (0, 2)
    (2, 5)
    (3, 7)
  };
  \draw[name path=circle] (2, 5) circle[radius=2];
  \draw[name intersections={of=phantom and circle}]
    (intersection-1) -- (hw.south west)
    -- (hw.north east) -- (intersection-2)
  ;
\end{tikzpicture}
\end{document}

Now, we hide the plot of the question and the node by putting them on the discarded layer:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{intersections}

\begin{document}
\begin{tikzpicture}
  \pgfdeclarelayer{discard}
  \begin{pgfonlayer}{discard}
    \path (2, 5) node (hw) {Hello World}; % or node[draw]
    \path[name path=phantom] plot coordinates { % or \draw
      (0, 2)
      (2, 5)
      (3, 7)
    };
  \end{pgfonlayer}
  \draw[name path=circle] (2, 5) circle[radius=2];
  \draw[name intersections={of=phantom and circle}]
    (intersection-1) -- (hw.south west)
    -- (hw.north east) -- (intersection-2)
  ;
\end{tikzpicture}
\end{document}

Because the discarded layer is not added to the output file, the PDF file size shrinks from 11696 to 1198 bytes (both: pdflatex with \pagestyle{empty}).