Home How can i determine which y-axis is on the right and left in this dual axis bar chart?

Questions

How can i determine which y-axis is on the right and left in this dual axis bar chart?

December 14, 2022

im currently trying to plot some bar charts where there are two values for each groups. As the values are far from each other i’ve made two axis.

However, i would like the "Accumulated RFU" y-axis to be on top instead of "Accumulated OD600", but so far it’s on the bottom.

My script looks like this for the plot:
ggplot(mean_sum_long, aes(x = Nitrogen_Source, y = value, fill = name)) +
  geom_col(position="dodge",alpha=0.8) +
  scale_y_continuous(
    name = "Accumulated RFU",
    sec.axis = sec_axis(~./coeff, name="Accumulated OD600"))+
  scale_fill_manual(values = c("green", "tomato"))+
  theme(axis.text.x = element_text(angle = 30, vjust = 0.5))+
  coord_flip()

coeff being 5000.

Data:

structure(list(Nitrogen_Source = structure(c(1L, 1L, 2L, 2L, 
3L, 3L), levels = c("Negative Control", "N-Phthaloyl-LGlutamic Acid", 
"D,L-α-AminoCaprylic Acid", "α-Amino-NValeric Acid", "L-Tryptophan", 
"Methylamine", "L-Cysteine", "Gly-Met", "Ethylenediamine", "Ala-His", 
"L-Tyrosine", "Ala-Leu", "Formamide", "D-Lysine", "D-Galactosamine", 
"Nitrate", "L-Histidine", "N-Acetyl-DMannosamine", "Ethylamine", 
"D-Mannosamine", "Histamine", "N-Acetyl-DGalactosamine", "Biuret", 
"D,L-Lactamide", "N-Acetyl-LGlutamic Acid", "Parabanic Acid", 
"Met-Ala", "L-Pyroglutamic Acid", "ß-Phenylethylamine", "Glucuronamide", 
"D-Glucosamine", "L-Glutamic Acid", "L-Glutamine", "Thymidine", 
"L-Isoleucine", "Xanthine", "Nitrite", "Ala-Asp", "Uridine", 
"ε-Amino-NCaproic Acid", "L-Citrulline", "D-Alanine", "Thymine", 
"L-Phenylalanine", "Ala-Glu", "Uric Acid", "γ-Amino-NButyric Acid", 
"Ammonia", "L-Aspartic Acid", "D-Aspartic Acid", "δ-Amino-NValeric Acid", 
"Ethanolamine", "L-Threonine", "Putrescine", "L-Methionine", 
"Allantoin", "L-Ornithine", "Ala-Thr", "Ala-Gln", "Cytosine", 
"Ala-Gly", "Agmatine", "L-Asparagine", "Inosine", "Guanosine", 
"Glycine", "N-Acetyl-DGlucosamine", "Adenosine", "Xanthosine", 
"L-Leucine", "Uracil", "L-Serine", "L-Homoserine", "Gly-Gln", 
"L-Proline", "L-Alanine", "Gly-Asn", "Urea", "Cytidine", "D-Glutamic Acid", 
"L-Arginine", "Gly-Glu", "Adenine"), class = "factor"), name = c("gfp_mean", 
"od_mean", "gfp_mean", "od_mean", "gfp_mean", "od_mean"), value = c(74851.5, 
41.29, 75282.5, 29.6650000000001, 76050, 58.755)), row.names = c(NA, 
-6L), class = c("tbl_df", "tbl", "data.frame"))

>Solution :

To achieve your desired result you have to use the reverse transformation, transform your gfp_mean values and of course switch the transformation for the secondary axis as well as the names.

Note: At least for the provided example data a coeff of 1250 seems more appropriate.

library(ggplot2)

coeff <- 1250

ggplot(mean_sum_long, aes(x = Nitrogen_Source, y = ifelse(name == "gfp_mean", value / coeff, value), fill = name)) +
  geom_col(position = "dodge", alpha = 0.8) +
  scale_y_continuous(
    name = "Accumulated OD600",
    sec.axis = sec_axis(~ . * coeff, name = "Accumulated RFU")
  ) +
  scale_fill_manual(values = c("green", "tomato")) +
  theme(axis.text.x.top = element_text(angle = 30, vjust = 0, hjust = 0)) +
  coord_flip()