Each code graph generated by the code analysis engine of radare can be stored in memory or disk using the 'R' command of radare.
[0xB7F73810]> R?
Usage: R[?] [argument] (TODO)
R ; list all RDBs loded in memory
R [rdb-file] ; load rdb file into memory
R -[idx] ; removes an rdb indexed
Rm [range] ; performs a merge between selected rdbs
RG [num] ; graph RDB number 'num'
Rd [a] [b] ; rdb diff. should generate a new rdb
In the same way it is possible to look for the differences between two different graph analysis of code by using the 'rdb diff' functionality of 'radiff -p' from the shell against two files generated by radare representing the graph structures or internally inside radare with the 'Rd' command.
This command will show the differences between these two graphs like new basic blocks, new edges, differences at byte level of the basic blocks to identify modified branches or so.
To generate an rdb file you just have to save the project using the 'Ps' command. This command will store the project file in ~/.radare/rdb/<project-file>. Take it from there to diff the code analysis with radiff.
TODO: To export an RDB file from IDA use the 'ida2rdb.idc' script that lives in the documentation directory of radare.