why dot file is generated rather jpg file in Ruby Graph Library (RGL)?

why dot file is generated rather jpg file in Ruby Graph Library (RGL)?

References

Reference NO 1

http://stackoverflow.com/questions/13658023/why-dot-file-is-generated-rather-jpg-file-in-ruby-graph-library-rgl

Reference NO 2

http://stackoverflow.com/questions/13659723/how-to-give-dynamic-input-for-rgldirectedadjacencygraph

Reference NO 3

http://stackoverflow.com/questions/15544632/can-we-convert-graph-into-set-in-ruby

Reference NO 4

http://mike.bailey.net.au/2011/02/json-with-ruby-and-rails/

Reference NO 5

http://repository.lib.ncsu.edu/ir/bitstream/1840.16/6081/1/etd.pdf

Reference NO 6

http://www.r-bloggers.com/

Reference NO 7

http://mirrors.ustc.edu.cn/cygwinports/x86/setup.ini

Reference NO 8

http://ftp.cs.pu.edu.tw/Linux/sourceware/cygwinports/setup.ini