If I have an array of X & Y data points, and I graph them via a scatter plot,
I will get a distribution around some regression line. If I take a line of
M=1 as my regression line, I can see that most of the dat awill fall along
this line, but there will be outliers. These can be described as fold
changes with respect to each other. A 2-fold change on a log base 10 chart
would correspond to lines drawn at y=x+0.3 & y=x-0.3. Does this help?