Do we explain a graph in results or discussion?
It may depend on your field or specific journal, but typically:
In the results section, describe the pattern quantitatively, with perhaps just a mechanistic matter-of-fact interpretation. 
In the discussion section, revisit the pattern but interpret qualitatively within the scope of your research question, tying it into your overall narrative. 

For publications with severe space limitations (e.g. Science, Nature), jump straight to qualitative interpretation and present quantitative results in the figure caption.