Are there any general rules or guidelines for creating abbreviations for words?

all-acronyms.com states that "Quest" can be used as an abbreviation for "Questionnaire".

I have seen this used before, though the word "quest" has an entirely different meaning then questionnaire does, so I generally avoid trying to abbreviate the word so that there is no confusion.

But in the context that you are using it (in a figure), perhaps if you labeled it as "quest." it would be suitable.

When in doubt, spell it out.

As for general rules, Wikipedia has the following:

If the original word begins with a capital letter, so should the abbreviation. (ex. Volume = Vol.) If the original word begins in lowercase, capitalization is not needed.

The use of periods differs between British English and American English.

• In British English, according to Hart's Rules, the general rule is that abbreviations terminate with a full stop (period), whereas contractions do not.
  Doctor (contraction) = Dr
  Professor (abbreviation) = Prof.
  The Reverend (contraction or abbreviation) = Revd or Rev.
  The Right Honourable (contraction and abbreviation) = Rt Hon.

• In American English, the period is usually added if the abbreviation might otherwise be interpreted as a word, but some American writers choose not to use one.

To form the plural of an abbreviation, a number, or a capital letter used as a noun, simply add a lowercase s to the end. (ex. Mind your Ps and Qs.)

To indicate the plural of the abbreviation of a unit of measure, the same form is used as in the singular. (ex. 1 min or 20 min.)