Table below x axis in ggplot

As an alternative approach to tackle this problem I simply set up the table as a second ggplot which I glue together with the major ggplot using patchwork.

## Data

Grade <- 1 : 20 
Case <-  rep(paste('case' , 1:5,sep = ''),4)
Number <- paste('n', 1:20 ,  sep = '')
Class <- c(rep('Class1',5) , rep('Class2',5) , rep('Class3',5) , rep('Class4',5))

se <- 0.2

df <- data.frame(Grade,Case ,Number, Class  , se)

library(patchwork)    
library(ggplot2)
library(tidyr)
library(dplyr)

## plot 

p1 <- ggplot(df, aes(x= factor(Case , levels = c('case1','case2' , 'case3' , 'case4','case5')) , y=Grade ,
               fill= Grade)) + 
  geom_bar(position="dodge", stat="identity",
           colour="black",
           size=.4) +    
  geom_errorbar(aes(ymin=Grade +se, ymax=Grade +se),
                size=.3,    
                width=.2,
                position=position_dodge(.9))+
  geom_linerange(aes(ymin = Grade , ymax = Grade +se),position=position_dodge(.9))+
  geom_text(aes(label=Number , y = Grade  + se + 1),data=df, position=position_dodge(0.9), size= 4) +
  ggtitle('Place a table below x axis')+
  facet_grid(~Class) + 
  xlab(NULL) + 
  ylab('Case Num') + 
  theme_gray()+
  theme(axis.text.x = element_blank())

p2 <- df %>% 
  mutate(First = as.integer(stringr::str_extract(Case, "\\d")),
         Second = First + 9,
         Third = Second + 9) %>% 
  pivot_longer(c(First, Second, Third), names_to = "layer", values_to = "label") %>% 
  ggplot(aes(x = Case)) +
  geom_text(aes(y = factor(layer, c("Third", "Second", "First")), label = label)) +
  labs(y = "", x = NULL) +
  theme_minimal() +
  theme(axis.line = element_blank(), axis.ticks = element_blank(), axis.text.x = element_blank(),
        panel.grid = element_blank(), strip.text = element_blank()) +
  facet_grid(~Class)

p1 / p2 +  plot_layout(heights = c(8, 1))

^{Created on 2020-05-23 by the reprex package (v0.3.0)}

EDIT: Tweak to get a more table like output by adding a geom_tile and removing the spacing between facets as well as setting expansion of x-axis to zero:

p2 <- df %>%
  select(Case, Class) %>% 
  mutate(First = letters[1:nrow(.)],
         Second = LETTERS[1:nrow(.)],
         Third = as.character(1:nrow(.))) %>% 
  pivot_longer(c(First, Second, Third), names_to = "layer", values_to = "label") %>% 
  ggplot(aes(x = Case, y = factor(layer, c("Third", "Second", "First")))) +
  # Add Table Style
  geom_tile(fill = "blue", alpha = .4, color = "black") +
  geom_text(aes(label = label)) +
  # Remove expansion of axsis
  scale_x_discrete(expand = expansion(mult = c(0, 0))) +
  labs(y = "", x = NULL) +
  theme_minimal() +
  theme(axis.line = element_blank(), axis.ticks = element_blank(), axis.text.x = element_blank(),
        panel.grid = element_blank(), strip.text = element_blank(), panel.spacing.x = unit(0, "mm")) +
  facet_grid(~Class)

p1 / p2 +  plot_layout(heights = c(8, 1))

^{Created on 2020-05-24 by the reprex package (v0.3.0)}

If I understand your requirement correctly, (as in my comment above), this may help you. You just need to name your graph and add the labels in loop and render outside the loop.

  ...
  theme(plot.margin = unit(c(1,1,1,6), "lines"),
        axis.text.x = element_text(size = 15))  +

  scale_x_discrete(labels = paste(1:5 , '\n' ,  10:15,  sep = '')) +
  coord_cartesian(clip = "off" , xlim = c(1, 5) )

  label = NULL
  ordinal <- c('first','second','third','fourth','fifth','sixth','seventh','eighth','ninth','tenth')
for (i in 1:5) {

  label <- paste(label, '\n', ordinal[i])



}
g1 <- g1 + geom_text(data = df[df$Class == 'Class1',],x = -1 , y = -3, 

          label= label , size = 4)

g1

This is what I get as a result: