Added an attribute for how spacing is computed for horizontal orienta…

[markwatabe]

Thanks for your interest in plotly.js!

Developers are strongly encouraged to first make a PR to their own plotly.js fork and ask one of the maintainers to review the modifications there. Once the pull request is deemed satisfactory, the developer will be asked to make a pull request to the main plotly.js repo and may be asked to squash some commits before doing so.

Before opening a pull request, developer should:

• git rebase their local branch off the latest master
• make sure to not git add the dist/ folder (the dist/ is updated only on verion bumps)
• write an overview of what the PR attempts to do.

Note that it is forbidden to force push (i.e. git push -f) to remote branches associated with opened pull requests. Force pushes make it hard for maintainers to keep track of updates. Therefore, if required, please git merge master into your PR branch instead of git rebase master.

…tion legends.