\documentclass[12pt]{standalone}

\usepackage{ucs}
\usepackage[utf8x]{inputenc}

\usepackage[T1]{fontenc}
\usepackage{textcomp}
\renewcommand{\rmdefault}{ugm}
\renewcommand{\sfdefault}{phv}
\usepackage[garamond]{mathdesign}
\usepackage{tikz}
\usetikzlibrary{positioning, shapes, arrows, shadows}

\begin{document}
\tikzset{
  observed/.style={circle, draw},
  partly observed/.style 2 args={draw, fill=#2, path picture={
      \fill[#1, sharp corners] (path picture bounding box.south west) -|
      (path picture bounding box.north east) -- cycle;},
     circle},
    unobserved/.style={draw, circle, fill=gray!40},
    residual/.style={draw, rectangle}
}

\tikzset{>=latex}

\begin{tikzpicture}
  \matrix [draw, below, font=\small, align=center, column sep=2\pgflinewidth, inner sep=0.4em, outer sep=0em, nodes={align=center, anchor=center}] at (current bounding box.south){
    \node[observed,label=right:observed] {}; \\
    \node[unobserved,label=right:automatically classified]{}; \\
    \node[residual,label=right:error term]{}; \\
  };
\end{tikzpicture}
\end{document}

