<p>I'm quite sure it's not a formal rule. Though it's clearly a sound principle (&amp; recall that answers are for all future readers, &amp; not just the original asker of a question) it can be hard to know where to draw the line in particular cases. At any rate people are entitled to down-vote according to their own opinion of which side of the line an answer falls on. Common practice here (again, not a formal rule) is for down-voters to explain their reason &amp; to reverse the down-vote if the answer is edited to remove that reason.</p>

<p>Note that the commenter in this case didn't explicitly say he down-voted your answer, so there's a possibility it may have been someone else.</p>

<p>My opinion on this particular case, for what it's worth, is that the evident heteroskedasticity in the example data set merits a mention at least, &amp; that your edit in response to criticism does improve your answer (+1).</p>
