Plot over Arbitrary Regions
In Mathematica, all plotting functions can operate over arbitrary implicitly defined regions.
In[1]:=  |  ContourPlot[Sin[x + Cos[y]], {x, -3, 3}, {y, -3, 3},
RegionFunction -> (1 < #1^2 + #2^2 < 9 &),
ColorFunction -> "BlueGreenYellow", BoundaryStyle -> Red] |
Out[1]=  |  |