If I notice an error in a figure after manuscript submission, could I be accused of falsification of data during review stage?
To answer your question, it sounds like the difference is so minor that this wouldn't be considered falsification but just a simple (and minor) mistake on your part.  You said yourself: the difference doesn't change the conclusions.

Addressing the issue is what the editor is for.  Contact them and point out that there was a bug that caused the figure to be generated slightly incorrectly (you are not the first person to have done this).  He/she will evaluate it and likely conclude the same as what you have: it is minor, does not affect the overall conclusions of the work, and will have you update it for final publication.  

The reviews are complete but it sounds like you haven't gotten the comments back.  It's likely that any discussion you have with the editor can conclude with you rolling this image in with the reviewer comments.

Don't overthink it.  Everyone involved in this process is human, despite carrying titles like "editor" and "reviewer."  Those folks are in those roles because they have experience and know that these types of things happen.