Display a sampling of field values as a two-dimensional bar graph.

Input parameters:

1. Fl → Field to sample.
2. Sl → Boundary of bar graph.
3. Dn → Number of samples per slice.
4. Mg → Optional function mapping the sampled values to bar heights.
5. Gr → Optional gradient mapping the sampled values to bar colours.

Output parameters:

1. Bg → Bar graph mesh.