Double bar-plot

I will use ggplot and dplyr/tidyr packages:

libs = c("ggplot2","tidyr","dplyr")
for(x in libs) { library(x,character.only=TRUE,warn.conflicts=FALSE) }

Suppose I have the following data:

df = data.frame(Day=1:10,Observed=floor((1:10-5)^2/4),Predicted=floor(10-(1:10-5)^2/4))

Creating the plot

# width of the bar (b/w zero and one)
wd = .6

df %>%
  # the main thing before plotting is that I transform my dataframe to long format
  gather(Variable,Count,-Day) %>%
  # here I decide the order in bar plots
  mutate(Variable=factor(Variable,levels=c("Observed","Predicted"))) %>%
  ggplot(aes(x=Day)) +
    geom_bar(aes(y=Count,fill=Variable),stat="identity",position=position_dodge(wd),width=wd) +
    # no margins (the same as xaxs="i",yaxs="i")
    coord_cartesian(expand=0,ylim=c(0,12)) +
    # breaks on x axis if needed
    scale_x_continuous(breaks=1:max(df$Day)) +
    # breaks on y axis if needed
    scale_y_continuous(breaks=seq(0,12,by=2)) +
    # position of the legend
    guides(fill=guide_legend(title.position="top", title.hjust = 0.5, title="variable")) +
    # axis labels
    labs(x="Day",y="Number of cases") +
    # if you want to make the background white, uncomment the following line
    #theme_bw() +
    theme(
      # remove minor grid if needed
      panel.grid.minor.x = element_blank(),
      #panel.grid.minor.y = element_blank(),
      # background of the legend is white
      strip.text.x = element_blank(),
      strip.background = element_rect(colour="white", fill="white"),
      legend.direction="vertical"
    )