Multiple boxplots

Question

In the following, you find a modified version of the boxplot from here (4 boxplots per data set instead of 2):

\documentclass[a4paper]{standalone}

\usepackage{pgfplots}
\usepgfplotslibrary{statistics}
\pgfplotsset{compat=1.8}

\begin{document}

\begin{tikzpicture}
\begin{axis}[
  boxplot/draw direction=y,
  ylabel={time (s)},
  height=8cm,
  boxplot={
      %
      % Idea: 
      %  place the 
      %  group 1 at 0,2 + 0,4 + 0,6 + 0,8
      %  group 2 at 1,2 + 1,4 + 1,6 + 1,8
      %  group 3 at 2,2 + 2,4 + 2,6 + 2,8
      %  group 3 at 3,2 + 3,4 + 3,6 + 3,8
      %  
      % in a formular:
      draw position={1/5 + floor(\plotnumofactualtype/4) + 1/5*mod(\plotnumofactualtype,4)},
      %
      % that means the box extend must be at most 0.2 :
      box extend=0.15,
  },
  % ... it also means that 1 unit in x controls the width:
  x=3cm,
  % ... and it means that we should describe intervals:
  xtick={0,1,2,...,50},
  x tick label as interval,
  xticklabels={%
      {Data set 1\\{\tiny off/on}},%
      {Data set 2\\{\tiny off/on}},%
      {Data set 3\\{\tiny off/on}},%
      {Data set 4\\{\tiny off/on}},%
  },
  x tick label style={
      text width=2.5cm,
      align=center
  },
  cycle list={{red},{blue},{green},{purple}},
]

\addplot
table[row sep=\\,y index=0] {
data\\
60\\
516\\
710\\
503\\
1253\\
};
\addplot
table[row sep=\\,y index=0] {
data\\
759\\
419\\
309\\
883\\
299\\
};
\addplot
table[row sep=\\,y index=0] {
data\\
516\\
480\\
1356\\
200\\
736\\
};
\addplot
table[row sep=\\,y index=0] {
data\\
684\\
340\\
700\\
325\\
377\\
};



\addplot
table[row sep=\\,y index=0] {
data\\
60\\
516\\
710\\
503\\
1253\\
};
\addplot
table[row sep=\\,y index=0] {
data\\
759\\
419\\
309\\
883\\
299\\
};
\addplot
table[row sep=\\,y index=0] {
data\\
516\\
480\\
1356\\
200\\
736\\
};
\addplot
table[row sep=\\,y index=0] {
data\\
684\\
340\\
700\\
325\\
377\\
};



\addplot
table[row sep=\\,y index=0] {
data\\
60\\
516\\
710\\
503\\
1253\\
};
\addplot
table[row sep=\\,y index=0] {
data\\
759\\
419\\
309\\
883\\
299\\
};
\addplot
table[row sep=\\,y index=0] {
data\\
516\\
480\\
1356\\
200\\
736\\
};
\addplot
table[row sep=\\,y index=0] {
data\\
684\\
340\\
700\\
325\\
377\\
};



\addplot
table[row sep=\\,y index=0] {
data\\
60\\
516\\
710\\
503\\
1253\\
};
\addplot
table[row sep=\\,y index=0] {
data\\
759\\
419\\
309\\
883\\
299\\
};
\addplot
table[row sep=\\,y index=0] {
data\\
516\\
480\\
1356\\
200\\
736\\
};
\addplot
table[row sep=\\,y index=0] {
data\\
684\\
340\\
700\\
325\\
377\\
};


\end{axis}
\end{tikzpicture}

\end{document}

Unfortunately, one boxplot is at a wrong position:

I expect this boxplot to be at position 3.2. Why is it on 3.0?

[EDIT] My workaround is to set that position manually:

\addplot+[boxplot={draw position=3.2}]

Answer 1

Addendum/New answer

With the release of TikZ/PGF v3.1 the below mentioned bug is fixed and thus OPs example now works as expected.

---

Old answer

This is because of an already known bug in the mod function. To circumvent the problem I adjusted a solution that was originally given by Jake.

% used PGFPlots v1.14
\documentclass[border=5pt]{standalone}
\usepackage{pgfplots}
    \usepgfplotslibrary{statistics}
    \pgfplotsset{compat=1.8}
    % borrowed from <https://tex.stackexchange.com/a/145967/95441>
    \pgfmathdeclarefunction{fpumod}{2}{%
        \pgfmathfloatdivide{#1}{#2}%
        \pgfmathfloatint{\pgfmathresult}%
        \pgfmathfloatmultiply{\pgfmathresult}{#2}%
        \pgfmathfloatsubtract{#1}{\pgfmathresult}%
        % replaced `0' by `5' to make it work for this problem
        \pgfmathfloatifapproxequalrel{\pgfmathresult}{#2}{\def\pgfmathresult{5}}{}%
    }
\begin{document}
\begin{tikzpicture}
    \begin{axis}[
        boxplot/draw direction=y,
        ylabel={time (s)},
        height=8cm,
        boxplot={
            %
            % Idea:
            %  place the
            %  group 1 at 0,2 + 0,4 + 0,6 + 0,8
            %  group 2 at 1,2 + 1,4 + 1,6 + 1,8
            %  group 3 at 2,2 + 2,4 + 2,6 + 2,8
            %  group 3 at 3,2 + 3,4 + 3,6 + 3,8
            %
            % in a formular:
            draw position={
                1/5 + floor(\plotnumofactualtype/4)
                  + 1/5*fpumod(\plotnumofactualtype,4)
            },
            % that means the box extend must be at most 0.2 :
            box extend=0.15,
        },
        % ... it also means that 1 unit in x controls the width:
        x=3cm,
        % ... and it means that we should describe intervals:
        xtick={0,1,2,...,50},
%        x tick label as interval,
%        xticklabels={%
%          {Data set 1\\{\tiny off/on}},%
%          {Data set 2\\{\tiny off/on}},%
%          {Data set 3\\{\tiny off/on}},%
%          {Data set 4\\{\tiny off/on}},%
%        },
%        x tick label style={
%          text width=2.5cm,
%          align=center
%        },
        cycle list={{red},{blue},{green},{purple}},
    ]
        \addplot table [row sep=\\,y index=0] {
            data\\
            60\\
            516\\
            710\\
            503\\
            1253\\
        };
        \addplot table [row sep=\\,y index=0] {
            data\\
            759\\
            419\\
            309\\
            883\\
            299\\
        };
        \addplot table [row sep=\\,y index=0] {
            data\\
            516\\
            480\\
            1356\\
            200\\
            736\\
        };
        \addplot table [row sep=\\,y index=0] {
            data\\
            684\\
            340\\
            700\\
            325\\
            377\\
        };

    % block 2
        \addplot table [row sep=\\,y index=0] {
            data\\
            60\\
            516\\
            710\\
            503\\
            1253\\
        };
        \addplot table [row sep=\\,y index=0] {
            data\\
            759\\
            419\\
            309\\
            883\\
            299\\
        };
        \addplot table [row sep=\\,y index=0] {
            data\\
            516\\
            480\\
            1356\\
            200\\
            736\\
        };
        \addplot table [row sep=\\,y index=0] {
            data\\
            684\\
            340\\
            700\\
            325\\
            377\\
        };

    % block 3
        \addplot table [row sep=\\,y index=0] {
            data\\
            60\\
            516\\
            710\\
            503\\
            1253\\
        };
        \addplot table [row sep=\\,y index=0] {
            data\\
            759\\
            419\\
            309\\
            883\\
            299\\
        };
        \addplot table [row sep=\\,y index=0] {
            data\\
            516\\
            480\\
            1356\\
            200\\
            736\\
        };
        \addplot table [row sep=\\,y index=0] {
            data\\
            684\\
            340\\
            700\\
            325\\
            377\\
        };

    % block 4
        \addplot table [row sep=\\,y index=0] {
            data\\
            60\\
            516\\
            710\\
            503\\
            1253\\
        };
        \addplot table [row sep=\\,y index=0] {
            data\\
            759\\
            419\\
            309\\
            883\\
            299\\
        };
        \addplot table [row sep=\\,y index=0] {
            data\\
            516\\
            480\\
            1356\\
            200\\
            736\\
        };
        \addplot table [row sep=\\,y index=0] {
            data\\
            684\\
            340\\
            700\\
            325\\
            377\\
        };
    \end{axis}
\end{tikzpicture}
\end{document}