add letters to dimension reduction fig

[jaybee84]

Purpose

closes issue #160

Directions for reviewers

Which areas should receive a particularly close look?

Is there anything that you want to discuss further?

Is the pull request ready for review?

Manuscript checklist

Unless otherwise noted above, this PR will be considered ready for review when all four items have been checked.

• [ ] All changes to text follow "one sentence per line" [Manubot instructions]
• [ ] All citations follow the Manubot citation instructions
• [ ] All changes or additions to tables follow the Manubot table instructions
• [ ] All figures follow the Manubot figure instructions
• [ ] All terms that need to be added to glossary have been added to #50
• [ ] There are no new spelling errors identified by the Manubot spellchecker

[AppVeyorBot]

AppVeyor build 1.0.392 for commit 3610c97d2379fcab073644e651756b7b9b0c48e1 is now complete.

Found 1 potential spelling error(s). Preview:

content/04.model-complexity.md:11:TODO...

The rendered manuscript from this build is temporarily available for download at:

• manuscript-1.0.392-3610c97.html
  • manuscript-1.0.392-3610c97.pdf
  • spelling-errors-1.0.392-3610c97.txt
  • spelling-error-locations-1.0.392-3610c97.txt