[fix] reference_line function throwing s4 class error for some users

[Schiano-NOAA]

There is an issue using ggplot_build function in the reference_line function where for a user it cause the function to fail and the following error:

Error in vapply(x, is.null, logical(1)) : 
  trying to get slot "data" from an object (class "ggplot2::ggplot_built") that is not an S4 object