Color facets in ggplot by continuous variable - r

Question

** Edited with Repeatable Data **
I have a data.frame with plots of growth over time for 50 experimental treatments. I have plotted them as a faceted 5x10 plot grid. I also ordered them in a way that makes sense considering my experimental treatments.
I ran a regression function to find growth rate in each treatment, and saved the slope values in another data frame. I have plotted the data, the regression line, and the value of growth rate, but I want to color the backgrounds of the individual faceted plots according to that regression slope value, but I can't figure out how to set color to call to a continuous variable, and especially one from a different df with a different number of rows (original df has 300 rows, df I want to call has 50 - one for each treatment).
My code is as follows:
Df:
df <- data.frame(matrix(ncol = 3,nrow=300))
colnames(df) <- c("Trt", "Day", "Size")
df$Trt <- rep(1:50, each=6)
df$Day <- rep_len(1:6, length.out=300)
df$Size <- rep_len(c(3,5,8,9,12,12,3,7,10,16,17,20),length.out = 300)
Regression function and output dataframe:
regression=function(df){
reg_fun<-lm(formula=df$Size~df$Day)
slope<-round(coef(reg_fun)[2],3)
intercept<-round(coef(reg_fun)[1],3)
R2<-round(as.numeric(summary(reg_fun)[8]),3)
R2.Adj<-round(as.numeric(summary(reg_fun)[9]),3)
c(slope,intercept,R2,R2.Adj)
}
library(plyr)
slopevalues<-ddply(df,"Trt",regression)
colnames(slopevalues)<-c ("Trt","slope","intercept","R2","R2.Adj")
Plot:
ggplot(data=df, aes(x=Day, y=Size))+
geom_line() +
geom_point() +
xlab("Day") + ylab("Size (μm)")+
geom_smooth(method="lm",size=.5,se=FALSE)+
geom_text(data=slopevalues,
inherit.aes=FALSE,
aes(x =1, y = 16,hjust=0,
label=paste(slope)))+
facet_wrap(~ Trt, nrow=5)
What I want to do is color the backgrounds of the individual graphs according to the slope value (slopevalues$slope) on a gradient. My real data are not just 2 values repeated, so I want to do this on a gradient of colors according to that value.
Any advice welcome.
enter image description here

Answer 1

You can use geom_rect with infinite coordinates to do this:
ggplot(data=df, aes(x=Day, y=Size))+
## This is the only new bit
geom_rect(
aes(fill = slope, xmin = -Inf, xmax = Inf, ymin = -Inf, ymax = Inf),
slopevalues,
inherit.aes = FALSE
) +
## New bit ends here
geom_line() +
geom_point() +
xlab("Day") + ylab("Size (μm)")+
geom_smooth(method="lm",size=.5,se=FALSE)+
geom_text(data=slopevalues,
inherit.aes=FALSE,
aes(x =1, y = 16,hjust=0,
label=paste(slope)))+
facet_wrap(~ Trt, nrow=5)