Add bottom-up and right-left orientations for Sankey diagrams

[wf-r]

What

See #7865

Adds two new flow directions to Sankey traces and gives the existing horizontal/vertical modes explicit, direction-named aliases, fully backward compatible.

orientation now accepts:

• h — horizontal, default (unchanged)
• left-right — sources left, flow rightward (synonym of h)
• right-left — sources right, flow leftward (new)
• v — vertical (unchanged)
• top-down — sources top, flow downward (synonym of v)
• bottom-up — sources bottom, flow upward (new)

Why

So far, h is always left→right and v is always top-down. Right-to-left and build-up (bottom-up) flows are common conventions with no equivalent today. This closes both gaps additively, without changing any existing rendering.

Backward compatibility

Purely additive. Existing figures using h/v render identically; the default is unchanged.

[camdecoster]

I'm going to change the target branch from master to our upcoming major version branch, v4.0. We've made some changes in the Sankey code and I'd like to add your changes against that. If you have time, it would be great to get this in for that version. I'll leave you a review tomorrow.

[wf-r]

Thanks for having a look (I have rebased on v4.0 for the moment)

[camdecoster]

This is a great addition and pretty simple to add. Could you please separate out the direction config into a second attribute? I added a suggestion. This will require changes to your code elsewhere, but I don't think it will be too much work. Let me know what you think

[camdecoster]

Instead of adding additional orientation values, let's add another attribute that covers the direction of the chart. This follows precedent with some existing attributes (polar.angularaxis.direction, pie.direction, etc.) and keeps two config options distinct.

        direction: {
            valType: 'enumerated',
            values: ['forward', 'reverse'],
            dflt: 'forward',
            description: [
                'Sets the direction of the flow along the `orientation` axis.',
                'With `forward` (the default), sources are on the left (horizontal) or top (vertical).',
                'With `reverse`, sources are on the right (horizontal) or bottom (vertical).',
            ].join(' ')
        }

[camdecoster]

Suggested change

	values: ['v', 'h', 'left-right', 'right-left', 'top-down', 'bottom-up'],
	values: ['v', 'h'],

[camdecoster]

Suggested change

	description: [
	'Sets the orientation of the Sankey diagram.',
	'`left-right` (synonym of the legacy value `h`) places sources on the left',
	'with the flow running rightward; `right-left` places sources on the right',
	'with the flow running leftward; `top-down` (synonym of the legacy value `v`)',
	'places sources at the top with the flow running downward; `bottom-up` places',
	'sources at the bottom with the flow running upward.'
	].join(' ')
	description: [
	'Sets the orientation of the Sankey diagram.',
	'With `h` (the default), the flow runs horizontally.',
	'With `v`, the flow runs vertically.',
	].join(' ')