Change `draw_line` and `draw_markers` to 'draw_marked_line`.

Exporter lets the base renderer farm out the appropriate calls to
`draw_markers` and `draw_lines`. Renderer's can reimplement the
`draw_marked_line` method if they wish to keep the line and marker
information bundled together.

If a renderer doesn't wish to do so, they should NOT reimplement the
`draw_marked_line` function, instead reimplementing both `draw_line` and
`draw_markers`.