Wolfram Technology Guide: Integrated Geometric Computing  previous | next 
Plot over Arbitrary Regions
In Mathematica, all plotting functions can operate over arbitrary implicitly defined regions.
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Click for copyable input
ContourPlot[Sin[x + Cos[y]], {x, -3, 3}, {y, -3, 3}, 

 RegionFunction -> (1 < #1^2 + #2^2 < 9 &), 

 ColorFunction -> "BlueGreenYellow", BoundaryStyle -> Red]
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