Fix regression in plot_effect()

description

In the previous version plot_effect() only showed two lines by default, not the four it did before that. This merge restores the original behaviour.

system details

• RStudio Version: 1.2.5033
• R Version: 3.5.2
• OS Version: Windows 10

checklist

• This merge includes bug fixes or changes in the functionality of the package (whether they are user-visible or not).
  • update documentation including examples (if necessary)
  • update and polish NEWS.md
  • update version in DESCRIPTION
  • update date in DESCRIPTION
• This merge introduces new features.
  • new features are documented
  • new features have tests
• no remaining TODO, FIXME, or debug prints anywhere in the source files
• devtools::document()
• devtools::check() without errors or warnings

DESCRIPTION

 Package: stressaddition
 Type: Package
 Title: Modeling Tri-Phasic Concentration-Response Relationships
-Version: 2.1.0
+Version: 2.1.0.9000
 Date: 2020-02-25
 Authors@R: person("Sebastian", "Henz", role = c("aut", "cre"), 
                   email = "sebastian.henz@ufz.de",