Citing unpublished figure from an ex colleague A former colleague has created a figure. The figure is not published and serves only as a mean for information sharing. I used to be a part of that research group and thus had access to it.

I published an article featuring a modified version of this figure, to explain my current experimental set-up. I did not cite the author of the figure as the figure was not found in any published material. I didn't know what to do - perhaps add them in Acknowledgments?

Have I done the wrong thing?
The correct thing to do is to cite them for the figure by name, if they will permit it. List the citation as "private communication". You should also probably indicate the it is "adapted from a private communication..."

You have committed plagiarism if you give the impression that you are the source. But it is easy to avoid with a citation. 

You can probably handle it in acknowledgements if you are clear about what you are acknowledging. Use language similar to the above.