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I've seen this done with a "watermark" that says (usually) "sample data" (kind of like this, from here, though that's a table rather than a chart). Think of the "draft" watermark you sometimes see...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/10267 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
I've seen this done with a "watermark" that says (usually) "sample data" (kind of like [this](http://www.ablebits.com/_img-blog/excel-insert-watermark/watermark-in-worksheet.png), from [here](http://www.ablebits.com/office-addins-blog/2013/09/19/insert-watermark-excel-worksheets/), though that's a table rather than a chart). Think of the "draft" watermark you sometimes see on documents; same idea. Saying something in the text (or figure caption) can be helpful, but this approach has the advantage of embedding the information directly in the graphic -- particularly useful if you're publishing online where somebody might save and reuse your image file, or if you're using the same chart in multiple sections of your document or multiple documents.