How to add percentage labels in a stacked bar keeping bars heights (R plotly)

Question

I am creating a stack bar in plotly. Each bar represents one year. I would like to keep heights of the bars and add labels representing share of a variable in a particular year (I would like a sum of percentages to be equal 100% for each bar).

Below a reproducible sample of code I am working with:

data <- data.frame("year" = c(2019, 2020, 2021),
                   "example_var1" = c(12872100, 69436460, 8129560),
                   "example_var2" = c(25589160, 19671712, 19648085))

vars = c("example_var1", "example_var2")

my_plot <- plot_ly(data, type = "bar")

# a loop is being used as vars names must be dynamic and depend on input data

for (k in vars) {
    
    my_plot <- my_plot %>%
        add_trace(x = ~ year, y = as.formula(paste0("~`", k, "`")), type = "bar", name = k,  text = "", textposition = 'auto') %>%
        layout (barmode = "stack")
}

my_plot

This is what I'm getting:

This is what I would like to get:

Any hint will be much appreciated. Thank you!

Answer 1

I'd reshape the data.frame to long format to calculate the percantage and to avoid the loop:

library(plotly)
library(data.table)

DT <- data.frame(
  "year" = c(2019, 2020, 2021),
  "example_var1" = c(12872100, 69436460, 8129560),
  "example_var2" = c(25589160, 19671712, 19648085)
)

setDT(DT)
DT <- melt(DT, id.vars = "year")
DT[, ratio := paste0(round(value / sum(value) * 100, digits = 0), "%"), by = year]

my_plot <-
  plot_ly(
    DT,
    x = ~ year,
    y = ~ value,
    type = "bar",
    name = ~ variable,
    text = ~ ratio,
    textposition = 'auto'
  ) %>%
  layout (barmode = "stack")

my_plot