No lumOff for Radar Charts

[oleibman]

Fix #661 (marked stale in 2018, but now reopened). That issue was already mostly resolved by many changes to Xlsx Chart Writer logic some time ago. However, a new problem popped up. PR #2950 added brightness logic to Xlsx Reader and Writer. That was done primarily for the benefit of scatter charts. Xlsx Writer writes two brightness properties lumMod and lumOff. These values are complete complementary (if you know one, you know the other), so I am not sure why both are needed, but my scatter chart testing indicated that they were. It turns out that Radar charts can also set brightness, but, if the writer specifies both lumMod and lumOff, the resulting chart is slightly off. There may be more to this, but that's all I can deal with for now - I suppress writing lumOff if we're writing a radar chart. If there are other problems in this area, I will wait for them to be reported. None of the existing radar chart samples used the brightness properties; however, the example attached to 661 did. It is added as a new Sample, and tests are added.

This is:

• a bugfix
• a new feature
• refactoring
• additional unit tests

Checklist:

• Changes are covered by unit tests
  • Changes are covered by existing unit tests
  • New unit tests have been added
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change and a link to the pull request if applicable
• Documentation is updated as necessary