mapping streamlines to scalar value

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Hi,

 

On a Gridded2DSet I am plotting a scalar value (density) as well as
streamlines for a 2D vector field (velocity).  Right now I have the density
mapped to the ZAxis and my streamlines lie in the z=0 plane.  My goal is to
have the streamlines lie on top of the density surface.  I came up with

 

(( x , y ) -> ( density )) -> ( u , v )

 

which clearly violates the FunctionType constructor.  Are there any easy
ways to accomplish this?  Thanks in advance.