first attempt at strip_text_x ha adjustment for facets

[jwhendy]

For review. Current status:

• hard codes info.ha as e.g. 'right' for testing purposes
• accepts 'left', 'center', and 'right' text args as well as float (matched approach here)
• anchor (justification) variable added and set to match info.ha (if str passed), or is 'left' if ha < 0.5 else 'right'

[has2k1]

ha and va get their values from

plotnine/plotnine/facets/strips.py

Line 70 in 4159f95

ha, va = "center", "bottom"

You can read those from the theme to get rid of the hard-corded values.

# This says, read the ha property of the strip_text_x themeable.
# If none has been set, use "center"
ha = theme.getp(("strip_text_x", "ha"), "center")
va = theme.getp(("strip_text_x", "va"), "bottom")

[has2k1]

Ignore the margin for now.

Then to make the [0, 1] range seem natural when used for positioning we have to take more control of the alignment.

Note that, when:

1. ha = "left" or ha = 0, the left of the text should be at the left of the strip box. i.e. we set horizontal alignment to left.
2. ha = "center" or ha = 0.5, the center of the text should be at the center of the strip box. i.e. we set horizontal alignment to center`.
3. ha = "right" or ha = 0, the right of the text should be at the right of the strip box. i.e. we set horizontal alignment to right.

That means when;

• ha = 0.2, the 0.2 * width_of_text point should be at 0.2 * width_of_strip_box.
• ha = 0.65, the 0.65 * width_of_text point should be at 0.65 * width_of_strip_box.
• ...

The way to do this is set the horizontal alignment to the left and calculate the x position with that in mind. This is what happens here

plotnine/plotnine/_mpl/_plotnine_tight_layout.py

Lines 211 to 214 in 4159f95

	box = bbox_in_figure_space(text, pack.figure, pack.renderer)
	x = params.left * (1 - f) + (params.right - box.width) * f
	text.set_x(x)
	text.set_horizontalalignment("left")