Made the data argument in Point's __init__method to be an optional argument by making it a boost::python::optional.
set (diagram-viewerSources
diagram.cpp
diagram-viewer-main.cpp)
set (diagram-viewerHeaders
diagram.h)
qt4_wrap_cpp (diagram-viewerMocSources ${diagram-viewerHeaders})
set (libraries ${libraries}
${Boost_SERIALIZATION_LIBRARY}
${Boost_PROGRAM_OPTIONS_LIBRARY}
${QT_LIBRARIES})
add_executable (diagram-viewer ${diagram-viewerSources}
${diagram-viewerMocSources})
target_link_libraries (diagram-viewer ${libraries})