add LCH diagram

Author: svgeesus

css-color-4/Overview.bs

@@ -1862,6 +1862,18 @@ Device-independent Colors: Lab and LCH</h2>
 	and H is the angle from the positive a axis,
 	with positive angles being more clockwise.
 
+	<figure>
+		<img src="images/CH-plane-wheel.svg">
+		<figcaption>
+			This figure shows the L=50 plane of the CIE Lab colorspace.
+			20 degree increments in CIE LCH are displayed as circles
+			at two levels of Chroma, 20 and 40.
+			All the 20 Chroma colors fit inside sRGB gamut,
+			some of 40 chroma are outside.
+			These out of gamut colors are visualized as grey, with a red warning outer stroke.
+		</figcaption>
+	</figure>
+
 	In Lab and LCH, if two colors have the same L value,
 	they appear to have the same visual lightness--
 	regardless of how different their hues are.