How to assign one conditional label per stacked bar (not per stack) on time series ggplot in facet grid - time-series

Question

I would love to have your input how to solve this issues.
I have a time series (x=date) ggplot with bars (y=cmass1), and my bars are on position="stack" using a factor (polymer) to display polymer types for each sample.
Each bar represents another sampling station (factor sample), and I would like to identify which bar is which station (factor "sample" in my data frame) and add the label above each bar.
I've come so far that I can show the "sample" labels, but then there appear multiple replicated labels (one for each data point in the stacked parts of the bar). But I need only one per bar!
Can someone help, how I can create one label per bar (above each bar) that identifies which sample it is (without manual annotation)?
Additional date display problem:
Also, on one date (11 November) I have two samples, and in this plot they are cumulated, but I would need them next to each - so two as stacked bars on one date (like with besides=TRUE, but that doesn't seem to work with stacked bars). Any help here?
I hope it is clear, and thanks so much for any help!
Cleo
The plot, where S1, S2 etc should be above each bar as a label
Dataframe view_partial
ggplot(Merg1[!(is.na(Merg1$campaign)),] ,
aes(date,y=cmass1,group_by(sample1), fill=polymer))+
geom_col(show.legend = T, alpha = 0.8, position="stack",
stat="identity")+
xlab("July 2021 November 2021 February 2022 July 2022") +
facet_grid(.~campaign, scales = "free_x",space = "free_x" )+
geom_point(aes(y=montsouris_mm*20 ),color="#A3009F",size=2.5, position="dodge")+
geom_point(aes(y=melun_mm*20 ),color="#558C8C",size=2.5, position="dodge")+
geom_point(aes(y=fauville_mm*20 ),color="#E8DB7D",size=2.5, position="dodge")+
scale_y_continuous(expand = c(0,0),limits=c(0, 1000), breaks = seq(0, 1000, by = 100),
sec.axis = sec_axis(~./20, name = "Precipitation (mm)",
breaks = seq(0, 50, by = 10))) +
scale_x_date(date_breaks = "1 day", labels = date_format("%d"))+
ylab(expression(bold(paste(~C[mass]~ (µg ~L^{-1}))))) +
theme(axis.text.x=element_text(color="black", size=12, angle=0, vjust=0.5),
axis.text.y=element_text(color="black", size=12, angle=0, vjust=0.5),
axis.ticks.x=element_blank(),
axis.title.x = element_text(size = 12,
color = "black",
face = "bold"),
strip.background = element_rect(color="white", fill="white", size=0.5,
linetype="solid"),
legend.text = element_text(color = "black",size = 12),
panel.grid.major = element_blank(),
panel.grid.minor = element_blank(),
panel.background = element_rect(size = 0.5,colour="#D4CDCB",
fill="white" ,linetype = 'solid'),
legend.key.height= unit(0.2, 'cm'),
legend.key.width= unit(0.4, 'cm'))+
scale_fill_manual(name = "", values = MPcol, breaks = c("abs","acrylic", "acrylic paints", "alkyd","epoxy", "pa", "pan_acrylic fibre", "pe", "polyester", "pp", "ps", "pu", "pvac", "pvc", "sbr", "vinyl copolymer"),
labels=c("Acrylonitrile butadiene styrene (ABS)","Acrylic", "Acrylic paint", "Alkyd", "Epoxy", "Polyamide (PA)", "Pan acrylic fibre", "Polyethylene (PE)", "Polyester (PET)", "Polypropylene (PP)", "Polystyrene (PS)", "Polyurethane (PU)", "Polyvinyl acetate (PVAC)", "Poly vinyl chloride (PVC)", "Styrene butadiene rubber (SBR)", "Vinyl copolymere"))+
theme(legend.position = "bottom",
legend.background = element_rect(fill = "#FFFCFB", # Background
colour = 1),
legend.title = element_text(family = "sans",
color = "black",
size = 10,
face = 2)) +
theme(panel.grid.minor = element_line(color = "#D4CDCB",
size = 0.15,
linetype = 2))+
theme(strip.text.x = element_text(size=14, face="bold",
vjust = 2, color="black")) +
#geom_text( label = label1, vjust = -1, position = position_dodge(0.90), size = 3, hjust=-0.5)+
labs(title = "Microplastics",
subtitle = "Mass Concentrations by Polymer Types",
caption = "Still missing S4 repeated samples and M1 in Campaign D. S4 in Campaign C cumulated repetaed samples. MPs >300µm are excluded, also natural particles are excluded.",
tag = "Fig. 5-1") +
theme(plot.title = element_text(family = "serif", # Font family
face = "bold", # Font face
color = 1, # Font color
size = 16, # Font size
hjust = 1, # Horizontal adjustment
vjust = 1, # Vertical adjustment
angle = 0, # Font angle
lineheight = 1, # Line spacing
margin = margin(5, 0, 0, 0)), # Margins (t, r, b, l)
plot.subtitle = element_text(family = "serif",
hjust = 1, size=14), # Subtitle customization
plot.caption = element_text(hjust = 0.25, size =11,
family = "serif",face = "italic"), # Caption customization
plot.tag = element_text(face = "italic"), # Tag customization
plot.title.position = "plot", # Title and subtitle position ("plot" or "panel")
plot.caption.position = "panel", # Caption position ("plot" or "panel")
plot.tag.position = "top") # Tag position