Showing different axis labels using ggplot2 with facet_wrap

Question

I have a time series with different variables and different units that I want to display on the same plot.

ggplot does not support multiple axis (as explained here),

Answer 1

Super late entry, but just solved this for myself...A super simple hack that lets you retain all strip.text is to just enter a bunch of spaces in ylab(" Voltage (V) Current (A)\n") in order to put the title across both plots. You can left-justify using axis.title.y = element_text(hjust = 0.25) to align everything well.

library(tidyverse)
library(reshape2)
x <- seq(0, 10, by = 0.1)
y1 <- sin(x)
y2 <- sin(x + pi / 4)
y3 <- cos(x)

my.df <-
    data.frame(
        time = x,
        currentA = y1,
        currentB = y2,
        voltage = y3
    )
my.df <- melt(my.df, id.vars = "time")
my.df$Unit <- as.factor(rep(c("A", "A", "V"), each = length(x)))

ggplot(my.df, aes(x = time, y = value)) + 
    geom_line(aes(color = variable)) + 
    facet_wrap( ~Unit, scales = "free_y", nrow = 2) +
    ylab("             Voltage (V)                                          Current (A)\n") +
    theme(
        axis.title.y = element_text(hjust = 0.25)
    )
ggsave("Volts_Amps.png",
       height = 6,
       width = 8,
       units = "in",
       dpi = 600)