Added a custom colour plotting example.
Review changes: various readability improvements.
Further review improvements.
Reduced scope for greater clarity.
Reverted unconnected change to custom-agg example (moved to PR #1040).
Review changes.
pep8 fix
Review: rewording.
pep8 fix.
More review suggestions.
Merge pull request #1040 from pp-mo/colours_eg

Added a custom colour plotting example.