Unit tests for common code

[aendra-rininsland]

While I think it's a bit too ambitious to consider trying to get this to 100% coverage, we should definitely find some way to test component code (axes, frames, etc.) so we can ensure changes don't add regressive bugs.

Add tests to the following; get above ≅90% coverage (where applicable)

• g-chartcolour
• g-chartframe
• g-axis
• g-legend

[sdbernard]

I think Tom has tests set up on g-chartframe and g-chartcolors

[aendra-rininsland]

@sdbernard I just looked at both of those, g-chartframe has one functional test (estimated code coverage is ~5-10%), and g-chartcolour only has a test stub (0% coverage).

[aendra-rininsland]

I've added code coverage reports to everything.