Enhancing the plots from another person's research paper
If you don't like how they have plotted the data, or there isn't sufficient resolution in their graphic (e.g., they used a PNG rather than PDF so that blowing up the document doesn't help resolution) then the only real solution is to ask the authors for a copy of their data, as suggested in the comments above.

You also may consider that if this does not work, the ugliness of their figure is not your responsibility: really, you are only including it as a courtesy to the reader, in order to make your document more self-contained.