assign colors to categories in stacked area graph ggplot

Question

I have to do several stacked area graphs in R with a common list of categories, but all the categories won't be present in all the graphs. So I created a vector assigning a colour to each category and used it in scale_fill_manual. It seems to work fine but the first category remains blanked. Anyone who know how to solve it?

An example (sort) of my data and the code I have used:

df <- structure(list(t = c(4, 8, 12, 4, 8, 12, 4, 8, 12, 4, 8, 12, 
4, 8, 12, 4, 8, 12, 4, 8, 12, 4, 8, 12, 4, 8, 12, 4, 8, 12, 4, 
8, 12, 4, 8, 12, 4, 8, 12, 4, 8, 12, 4, 8, 12, 4, 8, 12, 4, 8, 
12, 4, 8, 12, 4, 8, 12, 4, 8, 12, 4, 8, 12, 4, 8, 12, 4, 8, 12
), Orden = structure(c(1L, 1L, 1L, 2L, 2L, 2L, 5L, 5L, 5L, 6L, 
6L, 6L, 10L, 10L, 10L, 11L, 11L, 11L, 12L, 12L, 12L, 13L, 13L, 
13L, 14L, 14L, 14L, 15L, 15L, 15L, 7L, 7L, 7L, 8L, 8L, 8L, 16L, 
16L, 16L, 17L, 17L, 17L, 18L, 18L, 18L, 19L, 19L, 19L, 21L, 21L, 
21L, 22L, 22L, 22L, 23L, 23L, 23L, 24L, 24L, 24L, 25L, 25L, 25L, 
27L, 27L, 27L, 28L, 28L, 28L), .Label = c("Chaetocerotanae incertae sedis", 
"Corethrales", "Coscinodiscales", "Fragilariales", "Leptocylindrales", 
"Licmophorales", "Melosirales", "Naviculales", "Rhaphoneidales", 
"Rhizosoleniales", "Surirellales", "Thalassionematales", "Thalassiosirales", 
"Triceratiales", "Otras diatomeas centrales", "Otras células o cadenas lineales", 
"Cadenas de células pequeñas", "Otras diatomeas pennadas", "Otras diatomeas", 
"Actiniscales", "Dinophysiales", "Gonyaulacales", "Hemiaulales", 
"Noctilucales", "Peridiniales", "Prorocentrales", "Pyrocystales", 
"Otros dinoflagelados", "Appendicularia", "Choreotrichida", "Ciliophora", 
"Cirripedia", "Coccolithophores", "Copepods", "Cyanobacteria", 
"Dictyochales", "Fish egg", "Others", "Radiozoa", "Tintinnids", 
"Foraminifera"), class = "factor"), percentage = c(0.001, 0.002, 
0.005, 0, 0, 0, 0, 0.003, 0.001, 0.003, 0, 0, 0.033, 0.373, 0.169, 
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0.004, 0.017, 0.015, 0, 0, 
0, 0, 0, 0.002, 0.239, 0.245, 0.282, 0.681, 0.243, 0.382, 0.018, 
0, 0.039, 0, 0.001, 0, 0, 0.001, 0, 0.003, 0.044, 0.007, 0, 0, 
0, 0, 0.001, 0, 0.003, 0.019, 0.002, 0, 0, 0, 0.016, 0.051, 0.096
)), row.names = c(NA, -69L), class = c("tbl_df", "tbl", "data.frame"
))

orders_colours <- c("Chaetocerotanae incertae sedis" = "#595959", "Corethrales" = "#A6A6A6", "Coscinodiscales" = "#D9D9D9", "Fragilariales" = "#C5B8D0", "Leptocylindrales" = "#A18BB3", "Licmophorales" = "#775C8E", "Melosirales" = "#533569", "Naviculales" = "#251642", "Rhaphoneidales" = "#0A1E3E", "Rhizosoleniales" = "#123A74", "Surirellales" = "#1D81A2", "Thalassionematales" = "#004765", "Thalassiosirales" = "#53AAC9", "Triceratiales" = "#47C1B2", "Otras diatomeas centrales" = "#339A9B", "Otras células o cadenas lineales" = "#1C5558", "Cadenas de células pequeñas" = "#257085", "Otras diatomeas pennadas" = "#005D71", "Otras diatomeas" = "#163E4A", "Actiniscales" = "#FFBB7F", "Dinophysiales" = "#FFDC6C", "Gonyaulacales" = "#FFFBB1", "Hemiaulales" = "#FFE59C", "Noctilucales" = "#FFA126", "Peridiniales" = "#E65340", "Prorocentrales" = "#CC3E2F", "Pyrocystales" = "#731813", "Otros dinoflagelados" = "#390B09")

ggplot(df, aes(fill=Orden, y=percentage, x=t)) + 
  geom_area() +
  scale_fill_manual( values = orders_colours) +
  theme_light (base_size = 12, base_family = "Times")+
  theme(legend.position="bottom") +
  xlab("") + 
  ylab("")

And what I get is this kind of graph. In this case the percentage of "Chaetocerotanae incertae sedis" is too smal to be evident in the graph, however, as you can see in the legend it has no colour, although it should be dark grey...

Thanks in advance for the help!