The interactive shell is based on blockdiag. So, it already has capability of saving as PNG.
but I do not provide the feature.
because GAE does not support to install Pillow library by user.
And another reason is that generating PNG image is a heavy process.
if I provide the feature, it wastes CPU times and it causes service termination from depletion of free time of GAE.
Perhaps, moving to heroku resolve these problems.
but I don't have any time for that.