This iteration lowers the opacity of the points to give some idea about the density of points, and perhaps the strength of a correlation. The darker, more opaque areas are where multiple .3 opacity points are plotted on top of each other.
// scatterplot points
d3.select(this).selectAll("circle")
.data(data)
.enter()
.append("circle")
.attr("r", 2)
.style("fill", colors(row + col))
.style("fill-opacity", .3)Forked from Correlation Matrix III by micahstubbs
Originally inspired by the Simple Correlation Matrix block from emeeks