A graph is drawn in the style of "density" plot. This is similar to the "circle" style except for colors of circles. Closed circles are used in the "density" style inspite of open circles in the "circle" style.
In the "density" style, the color is related to the value of z relative to the maximum and minimum of the plot range of the z-axis. The color of a filled circle is a blend of two colors selected in the "Data Selector" window shown by [Data]-[Select Column]. One color corresponds to the z minimum and the other does the minimum. A data point whose z-value is somewhere between the maximum and the minimum will have a color made from the two original colors with a ratio proportional to (z-zmin)/(zmax-zmin).