V3.1.0

Closes #57

See merge request !35

closes #57
Use the names as default values.
NA will draw only the axes.
Update documentation.

because that info is available on CRAN anyway

Fixes to tests and package checks

Closes #53, #52, and #54

See merge request !33

According to CRAN rules all exported functions must specify a return value.

Update readme

Closes #47

See merge request !32

Release candidate

See merge request !31

version 3.0.0

Closes #24, #27, #36, #37, #38, #39, #40, #41, #43, and #45

See merge request !30

closes #38