The plumb_line module annotates the top surface of
an irregular 3D field, with red cross hairs over each node.
The cross hairs are pickable, and drive a line along
the remaining axis, through the 3D space. The (I,J) index
for the picked point is shown and can be controlled with the
X-Y integer dials, and are output on two integer output ports.
This can be used to control modules such as "orthogonal slicer"
to create a cutting sheet or a graph plot of the selected
data section. The picked cross hair changes color to white,
and a polyline following the mesh is drawn below.