Add undo button

[ufechner7]

It is very is to mess up a diagram, for example by selecting all components and click first "align horizontally" and then "align vertically".

Therefore a undo functionality would be very useful.

[bradcarman]

Thanks for the recommendation, I feel the same frustration sometimes. However, this is a very complex feature I think. If I can think of an easy way to implement I'll do it. For now the best I can recommend is to save often.