TikZ: adding nodes to a circular diagram from smartdiagram package

Question

This is what I would like to achieve:

My MWE is below:

\documentclass{article}
\pagestyle{empty}

\usepackage{lmodern}

\usepackage{smartdiagram}
\usetikzlibrary{shapes.geometric}
\usetikzlibrary{positioning}

\tikzstyle{every picture}+=[remember picture]

\smartdiagramset{%
circular distance=40mm,
font=\normalsize,
text width=20mm,
module minimum width=30mm,
module minimum height=30mm,
module shape=diamond,
arrow tip=to,
%uniform arrow color=true,
arrow color=gray!50!black,
border color=black,
set color list={white,white,white,white,white,white}
}

\tikzstyle{mystyle}=[align=center,fill=white,drop shadow,draw,thick,circle,overlay,minimum width=20mm,text width=25mm]

\begin{document}
~\vfill
\smartdiagram[circular diagram]{a, b, c, d, e, f}
\tikz \node (a) [mystyle,above=10mm of module1] {Start}; 
\tikz \node (b) [mystyle,right=10mm of module6] {Finish};
\begin{tikzpicture}[overlay,>=to]
\path[->,gray!50!black,line width=3pt,fill=gray!50!black] (a) edge (module1);
\path[->,gray!50!black,line width=3pt,fill=gray!50!black] (module6) edge (b);
\end{tikzpicture}
\vfill
\end{document}

I would achieve a similar result in a more intuitive way. As you can see I needed to go inside smartdiagram.sty to know how the nodes were named, and then I need to use the overlay. Moreover, suggestions for start and finish are welcome.

Answer 1

With version 0.3 (already sent on CTAN and thus available on TeXLive in few days) of the package to create such a diagram is a bit easier.

The code:

\documentclass[12pt]{article} 
\usepackage{smartdiagram}
\usesmartdiagramlibrary{additions} % library to create annotations 
\usetikzlibrary{shapes.geometric}
\begin{document} 
\begin{center}
\smartdiagramset{%
 circular distance=40mm,
 text width=20mm,
 module minimum width=30mm,
 module minimum height=30mm,
 module shape=diamond,
 arrow tip=to,
 uniform arrow color=true,
 arrow color=gray!50!black,
 border color=black,
 uniform color list=white for 6 items, % new key to make colors uniform easily
 circular final arrow disabled=true, % new key to remove the final arrow
 additions={
   additional item offset=15mm,
   additional item shape=circle,
   additional item border color=black,
   additional item shadow=drop shadow,
   additional arrow color=gray!50!black,
 }
}
\smartdiagramadd[circular diagram]{a, b, c, d, e, f}{
above of module1/Start,right of module6/End}
\smartdiagramconnect{-to}{additional-module1/module1}
\smartdiagramconnect{-to}{module6/additional-module2}
\end{center}
\end{document}

The result:

This method does not suffer anymore of the limitation on the number of additions that can be added.

---

First version

First of all a disclaimer: the solution I provide works if and only if one has to add just a start and an end to the diagrams.

It provides a new command \smartdiagramadd able to set the usual diagram plus the start/end: it receives 3 arguments:

1. type of diagram
2. list of elements in the diagram
3. position/text of the start, position/text of the end (for example: above/Start,right/End).

It can be used as:

\smartdiagramadd[circular diagram]{a, b, c, d, e, f}{above/Start,right/End}

Moreover, the solution allows to customize the distance of the start/end modules by means of two new keys:

\pgfkeys{/smart diagram/.cd,
  add start module offset/.initial=10mm,
  add start module offset/.get=\addstartmodoffset,
  add start module offset/.store in=\addstartmodoffset,
  add end module offset/.initial=10mm,
  add end module offset/.get=\addendmodoffset,
  add end module offset/.store in=\addendmodoffset,
}

They should be used always within \smartdiagramset; for example:

\smartdiagramset{add end module offset=20mm}

Now, the code:

\documentclass{article}
\pagestyle{empty}
\usepackage{lmodern}
\usepackage{smartdiagram}
\usetikzlibrary{shapes.geometric}
\usetikzlibrary{positioning}
\pgfkeys{/smart diagram/.cd,
  add start module offset/.initial=10mm,
  add start module offset/.get=\addstartmodoffset,
  add start module offset/.store in=\addstartmodoffset,
  add end module offset/.initial=10mm,
  add end module offset/.get=\addendmodoffset,
  add end module offset/.store in=\addendmodoffset,
}
\smartdiagramset{%
circular distance=40mm,
font=\normalsize,
text width=20mm,
module minimum width=30mm,
module minimum height=30mm,
module shape=diamond,
arrow tip=to,
uniform arrow color=true,
arrow color=gray!50!black,
border color=black,
set color list={white,white,white,white,white,white}
}
\tikzset{module start-end/.style={
  align=center,
  fill=white,
  drop shadow,
  draw,
  thick,
  circle,
  minimum width=20mm,
  text width=25mm}
}
\NewDocumentCommand{\smartdiagramadd}{r[] m m}{
\tikzstyle{every picture}+=[remember picture]
\smartdiagram[#1]{#2}
\begin{tikzpicture}[overlay]
\foreach \smitem [count=\xi] in {#2}  {\global\let\numitems\xi}
\StrCut{#3}{,}\startdiagram\enddiagram
% start
\StrCut{\startdiagram}{/}\pos\textitem
 \node (module1-start) [module start-end,\pos=\addstartmodoffset of module1] {\textitem}; 
 \path[->,gray!50!black,line width=3pt,fill=gray!50!black] 
  (module1-start) edge (module1);
% end
\StrCut{\enddiagram}{/}\pos\textitem 
 \node (module\numitems-end) [module start-end,\pos=\addendmodoffset of module\numitems] {\textitem};
 \path[->,gray!50!black,line width=3pt,fill=gray!50!black] 
  (module\numitems) edge (module\numitems-end); 
\end{tikzpicture}
}
\begin{document}
\begin{center}
\smartdiagramset{add end module offset=20mm}
\smartdiagramadd[circular diagram]{a, b, c, d, e, f}{above/Start,right/End}
\end{center}
\end{document}

which gives (similar to the picture provided by the OP, but the end module is far):

Disclaimer

This solution just work with smartdiagram version 0.2 which already loads xstring, otherwise the package should be loaded separately.