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How to use R to determine unknowns on an XY plot

x <- c(2.2,2.45,2.75,3.1,3.65)
y <- c(500,400,300,200,100)


mdata <- data.frame(x=x, y=log10(y))
my.fit <- lm(y ~ x, data=mdata)


unk <- data.frame(x=c(2.9, 3.3))
predict.lm(my.fit, unk, se.fit=T)

png(filename="mobility_plot.png", width=500, height=500)
plot(x, log10(y), pch=19, main="size vs. mobility", xlab="cm", ylab="size (bp)", yaxt="n")
abline(my.fit, col="grey")
# 2.381945 2.188279
segments(0, 2.38, 2.9, 2.38, lty="dashed", col="grey")
segments(0, 2.188, 3.3, 2.188, lty="dashed", col="grey")

segments(2.9, min(log10(y)), 2.9, 2.38, lty="dashed", col="grey")
segments(3.3, min(log10(y)), 3.3, 2.188, lty="dashed", col="grey")


axis(2, at = log10(y), labels = y)
text(2.25, 2.36, "241 bp", col="grey30")
text(2.25, 2.165, "154 bp", col="grey30")

dev.off()

Multiple plots at once

If you have multiple plots you can write a loop

Gel data 

size    Gel1    Gel2    Gel3    Gel4    Gel5    Gel6    Gel7    Gel8    Gel9
100     2.9     2.85    2.85    3       3       2.6     4.85    5.2     6.4
90      3.1     3       3       3.15    3.25    2.8     5.15    5.55    6.8
80      3.3     3.25    3.25    3.4     3.45    3       5.6     5.95    7.35
70      3.55    3.5     3.5     3.7     3.75    3.25    6.05    6.5     8
60      3.9     3.85    3.9     4.05    4.1     3.6     6.7     7.15    8.85
50      4.3     4.25    4.25    4.45    4.5     4       7.95    8       9.9
40      4.9     4.85    4.85    5.05    5.15    4.5     8.45    9.1     11.3
30      5.7     5.7     5.55    5.9     5.95    5.3     NA      NA      NA
20      6.95    6.9     6.85    7       7.2     6.4     NA      NA      NA

Unknowns 

Band    unk1    unk2    unk3    unk4    unk5    unk6    unk7    unk8    unk9
upper   6.05    5.9     6.25    3.6     3.9     3.3     5.05    5.6     7
lower   7.05    6.9     7.3     5.15    5.25    4.5     6.1     6.6     8.4


# read in a files of standards
stds <- read.table(file="stds.txt", sep="\t", header=T)
unks <- read.table(file="unks.txt", sep="\t", header=T)

y <- stds[,"size"]

# png(filename="mobility_plots.png", width=500, height=500)
pdf(file="mobility_plots.pdf")
# par(mfrow=c(3,3))
for(i in 2:ncol(stds)){

  mdata <- data.frame(x=stds[,i], y=log10(y))
  my.fit <- lm(y ~ x, data=mdata)

  unk <- data.frame(x=unks[,i])
  unk1 <- predict.lm(my.fit, unk, se.fit=T)$fit[1]
  unk2 <- predict.lm(my.fit, unk, se.fit=T)$fit[2]
  size1 <- sprintf("%.1f", 10^unk1)
  size2 <- sprintf("%.1f", 10^unk2)
  # cat(size1, " ", size2, "\n")

  plot(mdata[,"x"], log10(y), pch=19, main=paste(colnames(stds)[i], "  size vs. mob."), xlab="cm", ylab="size (bp)", yaxt="n")
  abline(my.fit, col="grey")
  axis(2, at = log10(y), labels = y)

  segments(0, unk1, unk[1,], unk1, lty="dashed", col="grey")
  segments(0, unk2, unk[2,], unk2, lty="dashed", col="grey")

  segments(unk[1,], min(log10(y)), unk[1,], unk1, lty="dashed", col="grey")
  segments(unk[2,], min(log10(y)), unk[2,], unk2, lty="dashed", col="grey")
  
  labxmarg <- max(stds[,i],na.rm=T) - 1
  
  text(labxmarg, 1.95, paste("u =",size1), col="grey30")
  text(labxmarg, 1.84, paste("l =",size2), col="grey30")
}
dev.off()