Draw connector lines below their points instead of on top

[cduvallet]

Summary

Connector lines drawn by allocate() went to the
center of each point, using matplotlib's default zorder (2), which
is above the default zorder for scatter/patch collections (1). This
made the line color show through the point marker instead of
tucking behind it.

This PR adds a linezorder param to allocate(). It defaults to None,
which resolves to a value below the point's zorder (further lowered
to stay below scatter_plot's own zorder if provided) so the point
visually caps the line where it meets it. An explicit linezorder
(or plot_kwargs={"zorder": ...}) overrides this.

Also adds an example to the demo notebook.

Before / after

Before:

After:

Test plan

• Verified visually in an isolated environment that lines render
  behind points by default, both with and without scatter_plot
  passed.
• Verified explicit linezorder and plot_kwargs={"zorder": ...}
  overrides are respected even when scatter_plot is provided.
• Added a demo section to demo.ipynb showing default vs.
  overridden linezorder behavior.

[ckjellson]

Thank you, good catch!

I like changing the default value, but I feel like adding more parameters is not necessary since one can change the setting via plot_kwargs (linewidth is probably also redundant...). Suggestions:

1. Remove the new parameter, but set zorder in line_kwargs to your default value unless the option has been specified in plot_kwargs.
2. Remove images in ./images from the commit.
3. Restore changes to demo.ipynb (I don't want it to contain too much stuff)

What do you think?

[cduvallet]

All great points, and implemented! I also updated the plot_kwargs docstring to (1) mention zorder and (2) make it explicit that plot_kwargs overrides provided params (e.g. linewidth).

As a user, I appreciate having linewidth as an easily accessible parameter, so while it is technically redundant I would keep it in :)