# Create a data frame for illustration purposes
df <- data.frame(period = rep(1:10, 3),
                 group = rep(LETTERS[1:3], each = 10),
                 value = sample(100, 30, replace = TRUE))

df$exp.value = exp(df$value)

# Split dataframe by group
 df_split <- split(df, df$group)

 # Plots of values in each group
 plots <- lapply(df_split, function(i){
   ggplot(data = i, aes(x = period, y = value)) + geom_line()
})

 # Plots of logged values in each group
 plots_exp <- lapply(df_split, function(i){
   ggplot(data = i, aes(x = period, y = exp.value)) + geom_line()
})

multiplot(plots[[1]], plots_log[[1]], cols = 1)