[Fix] plot_spawning_biomass when user wants to use DERIVED_QUANTITIES from SS3

[Schiano-NOAA]

Summary

An issue was identified in plot_spawning_biomass (and most likely our other plots) when wanting to access specific data. After some investigation, "DERIVED_QUANTITIES" is found in the converted dataset when era = NULL; however there was still an issue finishing the plot because era = NULL in the reference line results in an error. The issue was identified in #219

The fix:

• adjust reference line to take our era and use plot data instead

Additional changes

• took out a conditional section when era = NULL plotting horizontal break lines (this is doing too much and not needed)

@JoelRice-NOAA could you take a look at this and confirm this is working for you now? The best way to do this is the following:

remotes::install_github("nmfs-ost/stockplotr@fix-sb-plt-219")
# using your example file
albacore <- stockplotr::convert_output(output_file)
stockplotr::plot_spawning_biomass(
  dat = albacore,
  era = NULL # this is a key change
  ref_line = c("target" = 10e6),
  unit_label = "mt",
  scale_amount = 1e6,
  relative = TRUE,
  module = "DERIVED_QUANTITIES"
)

This is what I am getting from the example file in stockplotr:

plot_spawning_biomass(stockplotr::example_data, era = NULL)