Several sets of (x, y) points, with the Pearson correlation coefficient of x and y
for each set. Note that the correlation reflects the noisiness and direction of a
linear relationship (top row), but not the slope of that relationship (middle),
nor many aspects of nonlinear relationships (bottom). N.B.: the figure in the
center has a slope of 0 but in that case the correlation coefficient is undefined
because the variance of Y is zero.
copied from wikipedia