Making a spiral column chart with ggplot?

Question

I am trying to make a spiral side-by-side bar chart like the following examples:

The previous one is from Spiral barplot using ggplot & coord_polar (Condegram), but I couldn't make it work for my data.

Using ggplot, this is as far as I have gotten:

data <- read.table(text="year   group1  group2
1973    25939   27147
1978    21086   23108
1989    28401   24010
1995    34601   25457
2000    38672   28894
2007    40874   34926
2009    43892   38169
2013    48028   39270
2014    47289   39948
2015    48261   41913
2016    49814   42373
2017    50346   42818",header=T)

data$year <- as.character(data$year)

data <- data %>% gather(group, value, group1:group2)

ggplot(data)+
  geom_bar(aes(x=year, y=value, fill=group), stat="identity", position = "dodge") +
  coord_polar()

Which produces an ugly spiral bar chart

I'm not sure how to make the bottoms square and add the space the white spiral needs. Any help and explanation would be greatly appreciated!