\begin{figure}[t]
$
\begin{array}{llll}
(a)&
\includegraphics[width=0.4\columnwidth]{individual-homogeneity-distances}&
(b)&
\includegraphics[width=0.4\columnwidth]{individual-separation-distances}
\\
\end{array}
$
\caption[The distances used in the calculation of the homogeneity]{The distances
used in the calculation of the homogeneity (a) and the
separation (b) of cluster 1 in a hypothetical clustering (shown as lines
connecting individual cluster elements). Please note the differences of this
figure to Figures~\ref{fig:homogeneity-distances}
and \ref{fig:separation-distances}.}
\label{fig:individual-cluster-hom-sep-distances}
\end{figure}