ggplot2在一页上的多个图形之外的位置annotation_custom

Question

I have a plot doing bar plot for categorical variable (eg Group A, Group B, ...). I want to have multiple graphs on one page (something like this ). I want to create a text outside the plot using:

###### Plot three graphs together with one legend and one text on the right side

## this is the function that make one legend for multiple graphs
g_legend <- function(a.gplot){
  tmp <- ggplot_gtable(ggplot_build(a.gplot))
  leg <- which(sapply(tmp$grobs, function(x) x$name) == "guide-box")
  legend <- tmp$grobs[[leg]]
  return(legend)}

mylegend <- g_legend(p3.legend)

### combine three graphs with legend     
p <- grid.arrange(arrangeGrob(p1 , p2 , p3, mylegend, ncol = 4, widths = c(20, 20, 20, 11)))  ## vertical 

### create text to put outside this multi-graph plot
Text1 = textGrob(paste("An example text"))    
p1 = p + annotation_custom(grob = Text1,  xmin = 1, xmax = 2, ymin = 20, ymax = 30)

It gave me an error "Error in non-numeric argument to binary operator". Since x axis is categorical variable, so I thought the error was caused by xmax, xmin values cannot be assigned to categorical data. So I used p1 = p + annotation_custom(grob = Text1,xmin = -Inf, xmax = Inf, ymin = -Inf, ymax = Inf) . Again this same error.

I know how to do legend outside for multi-graph plot based on this reference: ggplot2: multiple plots with different variables in a single row, single grouping legend

The text outside example of Displaying text below the plot generated by ggplot2 is helpful. But this is for one graph, and I don't find it useful in my multi-graph plot situation.

Any idea about the reason of errors or how to position the annotation for multiple graphs one page with categorical x/y variables?

Answer 1

Something like this, where you have a factor variable on the x axis and use the factor integers to position the text grob?

require(ggplot2)
df <- data.frame(a = seq(0, 30, 6), group = as.factor(c("a", "b", "c", "d", "e", "f")))

Text1 <- textGrob("Test text")
ggplot(data = df, aes(x = group, y = a)) + 
  geom_bar(stat = "identity") +
  annotation_custom(grob = Text1,  xmin = 2, xmax = 3, ymin = 20, ymax = 22)