Clean up plot by related dx function documentation

[jnumainville]

Some function documentation is not clear, especially in regards to plot by functionality.

For example, for line:

width: A column or list of columns that contain width values.
  The value is used for a plot by on width.
  See width_map for additional behaviors.

The first line implies that the column itself can contain width values, but this isn't allowed at this time. The second sentence attempts to clarify this, but it would be better if the first sentence wasn't misleading in the first place.