PGF-PIE issues due to small values

Question

I have to include quite small values into my pie chart. Unfortunately the values are quite difficult to decipher. Is there an easy way to set small values (or all values) outside the pie and connect the values and the slices with a string?

\documentclass[
a4paper,
captions=tableheading
]
{scrbook}
%%%
\usepackage[
backend=biber %%%% !!! BIBER !!!
]{biblatex}
\usepackage{caption}
\usepackage{tikz}   
\usepackage{pgf-pie}        
\usepackage{pgfplots}
\usepackage{booktabs}                   
\usepackage{pgfplotstable}
\usepackage{siunitx}                            
\usetikzlibrary{patterns}
\usepgfplotslibrary{statistics} 
\sisetup{output-decimal-marker={,},group-separator={\,},}
\def\ScanPercentage#1\afternumber{\SI{#1}{\percent}}
%%%
\begin{document}
\begin{figure}
\textcolor{black!50}{\rule{\linewidth}{0.25pt}}
\begin{center}
\begin{tikzpicture}
\pie[
before number=\ScanPercentage,
after number={},
radius=3.00,  
text=legend, 
rotate=0,
pie values/.style={font={\scriptsize}},
pie legend/.style={font={\scriptsize}}
]
{
38.3/car,
25.0/aircraft,
13.3/boat,
6.7/horse,
1.7/goat,
0.8/ostrich,
14.2/na
}
\end{tikzpicture}
\end{center}
\textcolor{black!50}{\rule{\linewidth}{0.25pt}}
\vspace*{-10pt}
\caption[Pie Chart]
{\textbf{Pie Chart}}
\end{figure}
\end{document}  

In addition to that: Does a predefined "schema" exist, where tikz defines automatically grayscale "colours" for filling the different slices?

Answer 1

I only can answer the second part of the question. Torbjørn T. kindly informed me that there is a manual. I do not think one can easily move the numbers out without rewriting the macro. The second part of your question is easy: just add a list of colors after color=. In what follows, there is an ad hoc attempt of redefining the macro for the slices. The result is far from perfect. I am certainly not a TikZ expert, so what I am saying now has to be taken with a grain of salt. Yet I can't help from noting that many of the things done in that code could be done in an arguably more elegant and flexible way. I just do a minimal damage repair here. There is a key Move Out, which, if it is different from 1, will move the numbers out. The result is far from optimal, also because I continue to hard code things. I have seen much nicer pie charts on this site like e.g. this one, so to me the most promising way to go seems to be to adapt one of these.

\documentclass[
a4paper,
captions=tableheading
]
{scrbook}
\usepackage{caption}
\usepackage{tikz}   
\usepackage{pgf-pie}        
\usepackage{siunitx}                            
\sisetup{output-decimal-marker={,},group-separator={\,},}
\def\ScanPercentage#1\afternumber{\SI{#1}{\percent}}
%%%
\newif\ifOut
%\Debugtrue
\Outfalse
\pgfkeys{/pgf/.cd,
Move Out/.initial=1,
Move Out=1,
Minimum Angle/.initial=0}
\newcounter{myslice}
\makeatletter
\renewcommand{\pgfpie@slice}[8]{\stepcounter{myslice}
  \pgfmathparse{0.5*(#1)+0.5*(#2)}
  \let\midangle\pgfmathresult

  \path (#8) -- ++(\midangle:#5) coordinate(O);

  \pgfmathparse{#7+#5}
  \let\radius\pgfmathresult

  \pgfmathsetmacro{\SwitchOff}{ifthenelse(#2-#1<\pgfkeysvalueof{/pgf/Minimum
  Angle},1,0)}

  % slice
  \draw[line join=round, fill=#6, \style] (O) -- ++(#1:#7) arc (#1:#2:#7) -- cycle;

  \pgfmathparse{min(((#2)-(#1)-10)/110*(-0.3),0)}
  \let\temp\pgfmathresult
  \pgfmathtruncatemacro{\itest}{ifthenelse(\pgfkeysvalueof{/pgf/Move Out}==1,1,0)}
  \ifnum\itest=1
  \pgfmathsetmacro\innerpos{((max(\temp,-0.5)+0.8)*#7)}
  \else
  \pgfmathsetmacro\innerpos{\pgfkeysvalueof{/pgf/Move Out}*#7*(1+0.2*sin(90*\themyslice))}
  \fi
  \ifthenelse{\equal{\pgfpie@text}{inside}}
  {
    % label and number together
    \path (O) -- ++(\midangle:\innerpos) node
    {\scalefont{#3}\shortstack{#4\\\beforenumber#3\afternumber}};
  }
  {
    % label
    \iflegend
    \else
    \path (O) -- ++ (\midangle:\radius)
    node[inner sep=0, \pgfpie@text=\midangle:#4]{};
    \fi

    % number
    \ifnum\SwitchOff=1
    \else
     \ifnum\itest=1
      \path (O) -- ++(\midangle:\innerpos) node
      {\scalefont{#3}\beforenumber#3\afternumber};
     \else
      \path (O) -- ++(\midangle:\innerpos)  node (pie-N-\themyslice)
      {\scalefont{#3}\beforenumber#3\afternumber};
      \draw[latex-] (\midangle:{0.85*#7}) -- (pie-N-\themyslice);
     \fi
    \fi
  }
}
\makeatother
\begin{document}
\begin{figure}
\textcolor{black!50}{\rule{\linewidth}{0.25pt}}
\begin{center}
\begin{tikzpicture}
\pie[
before number=\ScanPercentage,
after number={},
radius=3.00,  
text=legend, 
rotate=0,
/pgf/Move Out=1.5,
/pgf/Minimum Angle=8,
color={gray!10,gray!20,gray!30,gray!40,gray!50,gray!60,gray!70},
pie values/.style={font={\scriptsize}},
pie legend/.style={font={\scriptsize}}
]
{
38.3/car,
25.0/aircraft,
13.3/boat,
6.7/horse,
1.7/goat,
0.8/ostrich,
14.2/na
}
\path (0,6);
\end{tikzpicture}
\end{center}
\textcolor{black!50}{\rule{\linewidth}{0.25pt}}
\vspace*{-10pt}
\caption[Pie Chart]
{\textbf{Pie Chart}}
\end{figure}
\end{document}  

As for the things raised in the comments:

1. I added a key Minimum Angle. Whenever the angle of a segment is smaller that this angle, the label will be supressed. For Minimum Angle=8, this kills 0,8% and 1,7%.
2. I added \path (0,6);, which moves the pie a bit down. If you use \path (0,7);, the pie will move further down.