The iso-shading diagram is a graphical expression of the shading factor table. It shows lines of some given shading factors, superimposed on the sun paths.
Blue lines also indicate the tangential limits of the plane (i.e., when the sun rays are parallel to the plane).
This diagram gives a synthetic evaluation of the shading distribution according to the season and the time-of day during the year.
The irregular look of the lines is due to the interpolations across discrete calculation points.
Remember that this loss factor applies to the beam component reaching the PV plane. When the incident angle is high, even high loss factors will act on very low irradiance component, giving rise to reasonable effects on the overall efficiency.