You can add circle areas to maps by making them out of a center coordinate and radius using
point: LatLng(51.50739215592943, -0.127709825533512),
Due to the nature of the Earth being a sphere, drawing lines perfectly requires large amounts of difficult maths that may not behave correctly when given certain edge-cases.
Avoid creating large polygons, or polygons that cross the edges of the map, as this may create undesired results.
Excessive use of circles may create performance issues.