r - ggplot2: Reversing secondary continuous x axis

Question

I am trying to reverse the secondary X axis on top of my ggplot.

ggplot(
  data=MasterTable, aes(x=Concentration, y=Signal, color=factor(Assay))) + 
  scale_x_continuous("Chemical 1", 
                     sec.axis = sec_axis(~ . *1, name = "Chemical 2"), scale_x_reverse(limits=c(400,0)))

If you remove the last section of the code (scale_x_reverse...) it makes a plot with a secondary that is identical to the bottom X axis. I have managed to reverse the bottom axis but this also reverses the top axis. I am looking to only reverse the top axis.

Any help on this would be really appreciated.

Thanks!

Answer 1

Here is a possibile solution:

MasterTable <- data.frame(Concentration=rep(c(0,50,100,200,300, 350, 400),2),
Signal=c(11800,13000,12000,12000,16000,15500,15570,11600,11700,8000,8000,6000,4000,3000),
Assay=rep(1:2,each=7))

library(ggplot2)
# Reverse Signal vector of the blue series (for Assay =1)
MasterTable$Signal[MasterTable$Assay==1] <- rev(MasterTable$Signal[MasterTable$Assay==1])

ggplot(data=MasterTable, aes(x=Concentration, y=Signal, color=factor(Assay))) + 
geom_line(lwd=1) + geom_point(size=3) + guides(color='none') +
scale_x_continuous('Chemical 1 (nM)', trans='reverse',
                   sec.axis = sec_axis(~ 400 - . , name='Chemical 2 (nM)'))