Put text in same coords with ggplot - r

Question

I'm trying to make a lot of graphs using ggplot2 script, and add some text (Lm equation and r2 value, using this function) for each graph.
The issue is that my x and y coordinates will be different between each graph.
With 'plot' function, you can convert 'plot' coords to 'figure' coords using cnvr.coord function, but in ggplot2 (grid base package), isn't functionally.
below and example (where "p" is a preexistent ggplot2 object) :
p <- p + geom_text(aes(X, Y, label = lm_eqn(lm(as.numeric(a$value) ~ as.numeric(a$date), a))))

Answer 1

I agree with shujaa. You can simply calculate where the function goes based on the range of your data. Using your link above, I've created an example:
library(ggplot2)
df1 <- data.frame(x = c(1:100))
df1$y <- 2 + 3 * df1$x + rnorm(100, sd = 40)
df1$grp <- rep("Group 1",100)
df2 <- data.frame(x = c(1:100))
df2$y <- 10 -.5 * df2$x + rnorm(100, sd = 100)
df2$grp <- rep("Group 2",100)
df3 <- data.frame(x = c(1:100))
df3$y <- -5 + .2 * df3$x + rnorm(100, sd = 10)
df3$grp <- rep("Group 3",100)
df4 <- data.frame(x = c(1:100))
df4$y <- 2 - 3 * df4$x + rnorm(100, sd = 40)
df4$grp <- rep("Group 4",100)
df <- list(df1,df2,df3,df4)
lm_eqn = function(df) {
m = lm(y ~ x, df);
l <- list(a = format(coef(m)[1], digits = 2),
b = format(abs(coef(m)[2]), digits = 2),
r2 = format(summary(m)$r.squared, digits = 3));
if (coef(m)[2] >= 0) {
eq <- substitute(italic(y) == a + b %.% italic(x)*","~~italic(r)^2~"="~r2,l)
} else {
eq <- substitute(italic(y) == a - b %.% italic(x)*","~~italic(r)^2~"="~r2,l)
}
as.character(as.expression(eq));
}
pdf("I:/test.pdf")
for (i in 1:4) {
text.x <- ifelse(lm(df[[i]]$y~1+df[[i]]$x)$coef[2]>0,min(df[[i]]$x),max(df[[i]]$x))
text.y <- max(df[[i]]$y)
text.hjust <- ifelse(lm(df[[i]]$y~1+df[[i]]$x)$coef[2]>0,0,1)
p <- ggplot(data = df[[i]], aes(x = x, y = y)) +
geom_smooth(method = "lm", se=FALSE, color="black", formula = y ~ x) +
geom_point()
p1 = p + geom_text(aes(x = text.x, y = text.y, label = lm_eqn(df[[i]])), parse = TRUE,hjust=text.hjust)
print(p1)
}
dev.off()