How to symbolize that figure is cropped?
What kind of a figure is that? If it is a plot, and you want to zoom in to a particular range of it, then you may want to include full plot, then a zoomed in one. For the latter, you may mention that "plot of y from x{a to b}" in the caption. Plus, it would be much better if you say a few lines about why this particular range is important to be emphasized this way, in between your text.

This is not a symbolization, but an effective one to crop something out of a drawing. Thus, I'd prefer this if I were to read your thesis.