Produce an inset in each facet of an R ggplot while preserving colours of the original facet content

Question

I would like to produce a graphic combining four facets of a graph with insets in each facet showing a detail of the respective plot. This is one of the things I tried:

Answer 1

Modifying off @user63230's excellent answer:

pp <- map(unique(data_frame$max_rep), function(x) {  
  plot2 + 
    aes(alpha = ifelse(max_rep == x, 1, 0)) +
    coord_cartesian(xlim = c(12, 14),
                    ylim = c(3, 4)) +
    labs(x = NULL, y = NULL) +
    scale_alpha_identity() +
    facet_null() +
    theme(
      strip.background = element_blank(),
      strip.text.x = element_blank(),
      legend.position = "none",
      axis.text=element_blank(),
      axis.ticks=element_blank()
    )
})

Explanation:

1. Instead of filtering the data passed into plot2 (which affects the mapping of colours), we impose a new aesthetic alpha, where lines belonging to the other replicate numbers are assigned 0 for transparency;
2. Use scale_alpha_identity() to tell ggplot that the alpha mapping is to be used as-is: i.e. 1 for 100%, 0 for 0%.
3. Add facet_null() to override plot2's existing facet_wrap, which removes the facet for the inset.

Everything else is unchanged from the code in the question.