  In agile software development, test code can considerably contribute to the
overall source code size. Being a valuable asset both in terms of verification
and documentation, the composition of a test suite needs to be well understood
in order to identify opportunities as well as weaknesses for further evolution.
In this paper, we argue that the visualization of structural characteristics is
a viable means to support the exploration of test suites. Thanks to general
agreement on a limited set of key test design principles, such visualizations
are relatively easy to interpret. In particular, we present visualizations that
support testers in (i) locating test cases; (ii) examining the relation between
test code and production code; and (iii) studying the composition of and
dependencies within test cases. By means of two case studies, we demonstrate
how visual patterns help to identify key test suite characteristics. This
approach forms the first step in assisting a developer to build up
understanding about test suites beyond code reading.
