How to center stacked percent barchart labels

Here's how to center the labels and avoid plotting labels for small percentages. An additional issue in your data is that you have multiple bar sections for each colour. Instead, it seems to me all the bar sections of a given colour should be combined. The code below uses dplyr instead of plyr to set up the data for plotting:

library(dplyr)

# Initial data frame   
df <- data.frame(Category, Brand, USD)

# Calculate percentages
df.summary = df %>% group_by(Brand, Category) %>% 
  summarise(USD = sum(USD)) %>%   # Within each Brand, sum all values in each Category
  mutate(percent = USD/sum(USD))

With ggplot2 version 2, it is no longer necessary to calculate the coordinates of the text labels to get them centered. Instead, you can use position=position_stack(vjust=0.5). For example:

ggplot(df.summary, aes(x=reorder(Brand, USD, sum), y=percent, fill=Category)) +
  geom_bar(stat="identity", width = .7, colour="black", lwd=0.1) +
  geom_text(aes(label=ifelse(percent >= 0.07, paste0(sprintf("%.0f", percent*100),"%"),"")),
                position=position_stack(vjust=0.5), colour="white") +
  coord_flip() +
  scale_y_continuous(labels = percent_format()) +
  labs(y="", x="")

With older versions, we need to calculate the position. (Same as above, but with an extra line defining pos):

# Calculate percentages and label positions
df.summary = df %>% group_by(Brand, Category) %>% 
  summarise(USD = sum(USD)) %>%   # Within each Brand, sum all values in each Category
  mutate(percent = USD/sum(USD),
         pos = cumsum(percent) - 0.5*percent)

Then plot the data using an ifelse statement to determine whether a label is plotted or not. In this case, I've avoided plotting a label for percentages less than 7%.

ggplot(df.summary, aes(x=reorder(Brand,USD,function(x)+sum(x)), y=percent, fill=Category)) +
  geom_bar(stat='identity',  width = .7, colour="black", lwd=0.1) +
  geom_text(aes(label=ifelse(percent >= 0.07, paste0(sprintf("%.0f", percent*100),"%"),""),
                y=pos), colour="white") +
  coord_flip() +
  scale_y_continuous(labels = percent_format()) +
  labs(y="", x="")

I followed the example and found the way how to put nice labels for simple stacked barchart. I think it might be usefull too.

df <- data.frame(Category, Brand, USD)

# Calculate percentages and label positions
df.summary = df %>% group_by(Brand, Category) %>% 
  summarise(USD = sum(USD)) %>%   # Within each Brand, sum all values in each Category
  mutate( pos = cumsum(USD)-0.5*USD)

ggplot(df.summary, aes(x=reorder(Brand,USD,function(x)+sum(x)), y=USD, fill=Category)) +
  geom_bar(stat='identity',  width = .7, colour="black", lwd=0.1) +
  geom_text(aes(label=ifelse(USD>100,round(USD,0),""),
                y=pos), colour="white") +
  coord_flip()+
  labs(y="", x="")