Tikz 3d trimetric view coordinate calculation bug

Question

I'm trying to draw a line in the x-y-plane and circles in the y-z-plane at constant x position.

Problem is, that the circle is far out of position (x-coordinate of center point of the circle is exactly the x-coordinate of the end of the line drawn from origin, but the circle is drawn elsewhere).

MWE:

\documentclass{standalone}
\usepackage{tikz}
%\usepackage{tikz-3dplot}
\usetikzlibrary{arrows,3d}


% see http://tex.stackexchange.com/a/48776/70600
 \makeatletter
 \tikzoption{canvas is xy plane at z}[]{%
     \def\tikz@plane@origin{\pgfpointxyz{0}{0}{#1}}%
     \def\tikz@plane@x{\pgfpointxyz{1}{0}{#1}}%
     \def\tikz@plane@y{\pgfpointxyz{0}{1}{#1}}%
     \tikz@canvas@is@plane
 }
 \makeatother 

% Drawing Views
\tikzstyle{trimetric}=[x={(0.926cm,-0.207cm)},y={(0cm,0.837cm)},z={(-0.378cm,-0.507cm)}]


\begin{document}
    \begin{tikzpicture}[trimetric, scale=0.1]

        \draw[dashed] (-1, 0,  0) -- (1, 0,  0);
        \draw[dashed] (0, -1,  0) -- (0, 1,  0);
        \draw[dashed] (0, 0,  -1) -- (0, 0,  1);

        \coordinate (O) at (0, 0, 0);
        \draw[-latex] (O) -- +(10, 0,  0) node [right] {$x$};
        \draw[-latex] (O) -- +(0,  10, 0) node [left] {$y$};
        \draw[-latex] (O) -- +(0,  0, 10 ) node [above] {$z$};  


        \begin{scope}[canvas is yz plane at x=120.0pt]
            \filldraw circle (.01); % center point
            \draw circle (91.62pt); % radius
        \end{scope}


        \begin{scope}[canvas is xy plane at z=0]
            \draw[] (0.0pt, 0.0pt) -- (120.0pt, 120.0pt);
        \end{scope}

    \end{tikzpicture}

\end{document}

Result:

Answer 1

The canvas ... at uses the internal \pgfpointxyz.

The latter function just takes numbers not dimensions; \pgfpointxyz{a}{b}{c} gives a times the x-vector plus b times the y-vector plus c times the z-vector.

In particular the units are ignored (rather than throwing an error).
If you write canvas is yz plane at x=120.0 you get the same result.

You need to specify x in your xyz system instead. As your x-vector has length .949cm you should write

x = 120.0*.949/28.4

to get the position corresponding to 120pt since 1cm=28.4pt.

\documentclass{standalone}
\usepackage{tikz}
%\usepackage{tikz-3dplot}
\usetikzlibrary{arrows,3d}


% see http://tex.stackexchange.com/a/48776/70600
 \makeatletter
 \tikzoption{canvas is xy plane at z}[]{%
     \def\tikz@plane@origin{\pgfpointxyz{0}{0}{#1}}%
     \def\tikz@plane@x{\pgfpointxyz{1}{0}{#1}}%
     \def\tikz@plane@y{\pgfpointxyz{0}{1}{#1}}%
     \tikz@canvas@is@plane
 }
 \makeatother 

% Drawing Views
\tikzstyle{trimetric}=[x={(0.926cm,-0.207cm)},y={(0cm,0.837cm)},z={(-0.378cm,-0.507cm)}]


\begin{document}
    \begin{tikzpicture}[trimetric, scale=0.1]

        \draw[dashed] (-1, 0,  0) -- (1, 0,  0);
        \draw[dashed] (0, -1,  0) -- (0, 1,  0);
        \draw[dashed] (0, 0,  -1) -- (0, 0,  1);

        \coordinate (O) at (0, 0, 0);
        \draw[-latex] (O) -- +(10, 0,  0) node [right] {$x$};
        \draw[-latex] (O) -- +(0,  10, 0) node [left] {$y$};
        \draw[-latex] (O) -- +(0,  0, 10 ) node [above] {$z$};  

        \begin{scope}[canvas is yz plane at x=120.0*.949/28.4]
            \filldraw circle (.01); % center point
            \draw circle (91.62pt); % radius
        \end{scope}

        \begin{scope}[canvas is xy plane at z=0]
            \draw[red,->] (0.0pt, 0.0pt) -- (120.0pt,0);
        \end{scope}

    \end{tikzpicture}

\end{document}