add ggplot2 loading to visc_empty skeleton.Rmd

[kelliemac]

Description

add ggplot2 loading to visc_empty skeleton.Rmd

Related Issues

addresses #251

Checklist

• This PR includes unit tests
• This PR establishes a new function or updates parameters in an existing function
  • The roxygen skeleton for this function has been updated using devtools::document
• I have updated NEWS.md to describe the proposed changes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.10%. Comparing base (b8969f4) to head (eba4663).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #256   +/-   ##
========================================
  Coverage    82.10%   82.10%           
========================================
  Files            8        8           
  Lines          352      352           
========================================
  Hits           289      289           
  Misses          63       63           

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