Add custom label in the middle in Alluvial plot

I have below Alluvial plot

library(ggplot2)
library(ggalluvial)
data = structure(list(val = c(10, 30, 15, 30, 40, 12, 10, 30, 15), name = c("A", 
"B", "C", "A", "B", "C", "A", "B", "C"), group = c("gr1", "gr1", 
"gr1", "gr2", "gr2", "gr2", "gr3", "gr3", "gr3")), row.names = c(NA, 
-9L), class = "data.frame")
data['val1'] = c('45', '23', '44', '55', '34', '55', '12', '34', '89')
  ggplot(data,
       aes(x = group, stratum = name, alluvium = name,
           y = val,
           fill = name, label = val)) +
  scale_x_discrete(expand = c(.1, .1)) +
  geom_flow(aes(fill = name), alpha = .3, width = 1/9) +
  geom_stratum(aes(color = name), alpha = .9, width = 1/9) +
  geom_text(position = "stack", aes(x = group, y = val, label = val1))

As you can see, that the labels are not vertically aligned at centre of each block. Is there any way to achieve this?

Thanks for your pointer

When you set position = "stack" in geom_text(), this is syntactic sugar for setting position = position_stack(vjust = 1, reverse = FALSE).

As you want your text vertically centered, you can explicitly call the position_stack() function, setting vjust = 0.5:

ggplot(data,
    aes(
        x = group, stratum = name, alluvium = name,
        y = val,
        fill = name, label = val
    )
) +
    scale_x_discrete(expand = c(.1, .1)) +
    geom_flow(aes(fill = name), alpha = .3, width = 1 / 9) +
    geom_stratum(aes(color = name), alpha = .9, width = 1 / 9) +
    geom_text(
        position = position_stack(vjust = 0.5), # only this line has changed
        aes(x = group, y = val, label = val1)
    )