I indeed read the link, but I did not completely understand what I had to do. (I am just a beginner in Rstudio) But I tried what you said I had to do and this is the result:
Bloodtypes <- data.frame(stringsAsFactors=FALSE,
Bloodtype = c("A-", "A+", "AB-", "AB+", "B-", "B+",
"O-", "O+"),
ABO = c("A", "A", "AB", "AB", "B", "B", "O", "O"),
Rhesus = c("-", "+", "-", "+", "-", "+", "-", "+"),
Percentage = c("7", "35", "0,5", "2,5", "1,3", "6,7", "7", "39,5"))
library(ggplot2)
ggplot(Bloodtypes, aes(x = Bloodtype, y = Percentage)) +
geom_col()

Created on 2019-04-25 by the reprex package (v0.2.1)
I hope the image works now.
Thank you for your help!