Little hack to allow users save their plots

This should allow users to save their plots for 'plot' and 'stacked' display, when they add "filename" argument, f.e.:
c.display('plot', filename="/home/honza/fig.png")
c.display('stacked', filename="~/plot.png")

When filename is not used, it's filled None and considered "Do not save plot".

Only temporary solution for now.