Isobologram

Input your data

x <- c(5, 4, 3, 2, 1, 0) # x mean 
x.ul <- c(0.1, 0.2, 0.5, 0.3, 0.2, 0.1) # x upper limit
x.ll <- c(0.2, 0.3, 0.5, 0.4, 0.3, 0.2) # x lower limit

y <- c(0, 2, 3, 4, 5, 6) # y mean
y.ul <- c(0.5, 0.4, 0.3, 0.2, 0.1, 0.1) # y upper limit    
y.ll <- c(0.1, 0.5, 0.1, 0.2, 0.3, 0.4) # y lower limit  

Calculate the IC50 line

intercept <- max(y)
slope <- -intercept/max(x)

Create plot

# Add data point 
plot(x, y, pch=16, ylim=c(0,7),xlim=c(0,7), col = "blue",
     xlab = "PEA", ylab = "Quercetin", main = "IC50 Isobolgram")
# Add CI
segments(x+x.ul, y ,x-x.ll, y) # x
segments(x, y+y.ul, x, y-y.ll) # y
# Add line
abline(intercept,slope, lwd = 2)