epic: clean up nanoplot implementation

[machow]

From pairing, we currently have pieces like boxplots in nanoplots teed up (#168). However, when we merged the nanoplot PR, we decided to punt on doing too much review / cleanup, in the name of getting it out.

Let's plan on cleaning up the nanoplot implementation a bit, so it's simpler to add things like boxplots. Below are some potential cleanups.

Avoiding unsetting arguments based on other arguments

We decide what goes into _construct_nanoplot_svg(), but currently it can quietly unset arguments.

great-tables/great_tables/_utils_nanoplots.py

Lines 532 to 540 in 7365aaa

	ref_area_tags = "" if ref_area_tags is None or show_reference_area is False else ref_area_tags
	area_path_tags = "" if area_path_tags is None or show_data_area is False else area_path_tags
	data_path_tags = "" if data_path_tags is None or show_data_line is False else data_path_tags
	zero_line_tags = "" if zero_line_tags is None else zero_line_tags
	bar_tags = "" if bar_tags is None else bar_tags
	ref_line_tags = "" if ref_line_tags is None or show_reference_line is False else ref_line_tags
	circle_tags = "" if circle_tags is None or show_data_points is False else circle_tags
	g_y_axis_tags = "" if g_y_axis_tags is None or show_y_axis_guide is False else g_y_axis_tags
	g_guide_tags = "" if g_guide_tags is None or show_vertical_guides is False else g_guide_tags

Do not fallback to unspecified behavior

For example, this if/else clause defaults to using a specific calculation (quartile 3).

great-tables/great_tables/_utils_nanoplots.py

Lines 420 to 421 in 7365aaa

	else:
	ref_line = _gt_q3(vals)

(Note that functions outside this one check that a calculation was specified, so in practice things work out okay)

Consolidate large branches inside _generate_nanoplot()

For example, the "Generate curved data line section" has two branches that seem similar to each other.

great-tables/great_tables/_utils_nanoplots.py

Lines 1030 to 1034 in 7365aaa

	#
	# Generate a curved data line
	#
	if plot_type == "line" and show_data_line and data_line_type == "curved":

It seems like if we had functions for curved_path() and polyline() that produced svg strings (or a data repr of SVG elements), that would help a lot. IMO everywhere that the variable data_path_tags gets used gives helpful hints for consolidating.

People can go overboard consolidating, so I think the key is finding the places where it cleans up readability / robustness the most.

[rich-iannone]

In speaking with @t-kalinowski we can do the following to improve this part of the codebase:

1. split _generate_nanoplot() into several smaller functions; try to reduce line length to ~20 lines
2. create a nanoplot spec which encapsulates all data and options in a class that initializes the values from input data and collected args
3. ensure there are reasonable contracts for each of the methods added