ob-ditaa.el calls Java VM to generate pictures. Unfortunately, Cygwin doesn't have native Java VM. In order to call the Windows native Java VM properly, all Cygwin paths pointing to ditaa.jar, the temporary input file and the output picture file must be changed to Windows native paths, for example, /home/yujie/ditaa.jar to d:/cygwin/home/yujie/ditaa.jar.