plotting_many_lines.py, vector_scatter: Something's still missing

[plotting_many_lines.py](https://github.com/enthought/mayavi/blob/master/docs/source/mayavi/auto/plotting_many_lines.py) is a nice demonstration of something that one would think, _naively_, would be the function of `vector_scatter`. Alas, there does not appear to be any way to tell `vector_scatter` to just use a good ole line instead of a glyph, and we're left with what feels like a clunky solution.

Why clunky? Well, one compelling use-case for the `plot_many_lines` code is visualizing the edge weights of a weighted graph. The idea here is to color each edge according to some scalar value. Except that that appears to be basically impossible to do in a clean way. In particular, the `scalar_scatter` to `surface` trick sets up a pipeline that only respects the _point_ color; the edges end up multicolored, an interpolation between the two endpoint colors.

So, AFAIK, the only way to get a set of lines with one solid color on each line is to modify the `plot_many_lines` code to create redundant point data -- two separate points -- for each line segment. This is highly inefficient since some scatter points may serve as endpoints to multiple lines, and generally ruins the elegance of the `scalar_scatter`-to-`surface` solution.

Question: What is the canonical way of plotting many line segments, each one with it's own solid color?