[R] How could I graph a special coordinate

[Klot Lee]

hi,

I'v got some data attached as apx.csv.  It shows the relationship about a
kind of enzyme and the temperature.
"yhf" and "xbt" are 2 kind of cowpea. "apx" is the kind of enzyme and "low"
means the lowest temperature intraday. "date" means 1 to 121 days.

What I want to show with it is how is "apx" changes with "low" on 2 kind of
cowpeas. And "date" is better not missing in graph.
I have made one by ggplot2 using apx2.csv (reshaped by melt).

apx<- read.csv("apx2.csv")
p<- ggplot(apx, aes(x= dat, y=apx, colour= factor(var)))
p + geom_line()+facet_wrap(~var,ncol=1, scales= "free_y")

The graph is NOT good enough. It is not easy to find the relationship
between "apx" and "low". How could I do better? Is it possible to show
"low" data as graph background color changing with "date"? Thank you.

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Stand Alone Complex