Creating Dual Axis plot (bar and line) - r

Question

I want to create a dual axis plot in ggplot R with a dual bar and line plot, like this one created in excel.
The y axis scales are different.
my data is as follows;
I've created a bar plot and line plot. But unsure on how to put them together (I've tried man various ways and they don't seem to work).
Here is my code for the bar plot.
inf_conc <- ggplot(data=data, aes(x=Day, y=inf)) +
geom_bar(stat="identity", width=0.4, color="red3", fill="red3") +
ggtitle("Influent Microplastic Concentration \n and Flow Rate") +
# \n splits long titles into multiple lines
xlab("Day") +
ylab("Microplastic Concentration (MPs/L)") +
scale_y_continuous(limits =c(0, 50), breaks = seq(0, 50, 5))
inf_conc + theme(axis.text = element_text(size = 20, colour = "black"),
plot.title = element_text(size =25, hjust = 0.5,
face = "bold"), axis.title = element_text(size = 20,
face = "bold", margin = 5))
inf_conc + theme(axis.text = element_text(size = 20, colour = "black"),
plot.title = element_text (size =25, hjust = 0.5, face = "bold"),
axis.title = element_text(size = 20, face = "bold", margin = 20))
and here is the code for the line plot:
inf_flow <- ggplot(data=data, aes(x=Day, y=flow, group = 1)) +
geom_line(stat = "identity", colour ="blue4") +
geom_point(colour ="blue4") +
ylab("Inlet flow L/s")+
xlab("Day")+
scale_y_continuous(limits=c(0,800), breaks = seq(0, 800, 100))
inf_flow + theme(axis.text = element_text(size = 20, colour = "black"),
plot.title = element_text (size =25, hjust = 0.5, face = "bold"),
axis.title = element_text(size = 20, face = "bold", margin = 5))
inf_flow + theme(axis.text = element_text(size = 20,
colour = "black"), plot.title = element_text (size =25, hjust = 0.5,
face = "bold"), axis.title = element_text(size = 20, face = "bold",
margin = 20))
Can anyone help with how I can get these onto one dual axis graph please.

Answer 1

GGplot doesn't make it especially easy, but you can do it:
library(ggplot2)
my_dat <- data.frame(
Day = paste("Day",rep(1:3, each=3), rep(c("(AM)", "(Midday)", "(PM)"), 3), sep= " "),
day_num = 1:9,
inf = seq(from = 13,to = 45, length=9),
flow = runif(9, 580, 740)
)
ggplot() +
geom_bar(data=my_dat, aes(x=day_num, y=inf, fill = "Influent Concentration"), stat="identity", width=.6) +
geom_line(data=my_dat, aes(x=day_num, y=flow*(50/800), colour="FLow Rate. L/s")) +
scale_fill_manual(values="red") +
scale_colour_manual(values="blue") +
scale_x_continuous(breaks=1:9, labels=my_dat$Day) +
scale_y_continuous(sec.axis = sec_axis(trans = ~.x*800/50, name = "Flow Rate L/S"), limits = c(0,50), name = "Influent. MPs/L") +
labs(fill="", colour="", x="") +
theme(legend.position="bottom",
axis.text.x = element_text(angle=45, hjust=1))
Created on 2023-01-17 by the reprex package (v2.0.1)
The main things you have to do are to
Transform the second-axis series to have the same range(ish) as the first-series axis. In your case, the excel graph had the second y-axis going from 0-800 and the first y-axis going from 0-50, so the transformation is simple, you multiply the second series values by 50/800.
In the scale_y_continuou() function there is an argument sec.axis which allows you to plot a second axis on the right-hand side of the plot. Here, you need to specify the trans argument to transform the values you're plotting back into the original values. That's what trans = ~.x*800/50 does.
EDIT: Modifying OP's code
I modified your code as much as I can without actually having the data. The picture of the data that you provided does not give enough information about the data, if you use dput(data) and post the results, I could likely help more. For now, try this:
inf_plot <- ggplot(data=data, aes(x=Day))+
geom_bar(aes(y=inf, fill="Influent conc"), stat = "identity", width=0.4)+
geom_line(aes(y=flow*(50/800), colour="flow rate"), size = 1.4, group=1)+
ggtitle("Influent Microplastic Concentration \n and Influent Flow Rate")+
xlab("\n\nDay") +
ylab("Microplastic Concentration (MPs/L)\n\n")+
scale_fill_manual(values="red4") +
scale_colour_manual(values="blue4") +
scale_y_continuous(sec.axis = sec_axis(~.*800/50, name = "Inlet flow rate (L/s)\n\n"), limits = c(0,50))
inf_plot + theme(axis.text = element_text( size = 20, colour = "black"),
plot.title = element_text (size =25, hjust = 0.5, vjust = 5, face = "bold"),
axis.title = element_text (size = 20, face = "bold"),
plot.margin =unit(c(1.5, 1.5, 1.5, 1.5), "cm"),
legend.position = "bottom")

Answer 2

The answer was a great help in how to transform my axis.
Initially produced the graph a slightly different way, but incorporated the same transformation of axis.
However, I can't seem to get the legend to appear at the bottom of the graph with the following code.
inf_plot <- ggplot(data=data, aes(x=Day))+
geom_bar(aes(y=inf, fill="Influent conc"), stat = "identity", width=0.4,
colour="red4", fill = "red4")+
ggtitle("Influent Microplastic Concentration \n and Influent Flow Rate")+
xlab("\n\nDay") +
ylab("Microplastic Concentration (MPs/L)\n\n")+
geom_line(aes(y=flow*(50/800), colour="flow rate"), size = 1.4, colour ="blue4", group = 1)+
scale_fill_manual(values="red4") +
scale_colour_manual(values="blue4") +
scale_y_continuous(sec.axis = sec_axis(~.*800/50, name = "Inlet flow rate (L/s)\n\n"), limits = c(0,50))
inf_plot + theme(axis.text = element_text( size = 20, colour = "black"),
plot.title = element_text (size =25, hjust = 0.5, vjust = 5, face = "bold"),
axis.title = element_text (size = 20, face = "bold"),
plot.margin =unit(c(1.5, 1.5, 1.5, 1.5), "cm"),
legend.position = "bottom")
enter image description here