Extract data from a PNG/TIFF
Sometimes it’s useful to be able to extract data from a published figure. If the figure isn’t a vector based format (for which the numeric data is probably still in the file), it’s possible to digitize the image with R, click the points and extract it that way. The digitize package is simple to use for this purpose…
If you save the top figure from here to a PNG called “Fig2a_Kreners2015” in your working directory, you can digitize the three lines and replicate the figure (or do other stuff if you so wish) as follows
library(digitize)
dig10 <- digitize("Fig2a_Kremers2015.png") # do the solid line
dig15 <- digitize("Fig2a_Kremers2015.png") # do the dashed line
dig20 <- digitize("Fig2a_Kremers2015.png") # do the dotted line
# rename the first variable
names(dig10)[1] <- names(dig15)[1] <- names(dig20)[1] <- "age"
plot(dig10, type = "l")
lines(dig15, lty = 2)
lines(dig20, lty = 3)
Easy huh? It just gets a bit laborious with many points… 🙂
Insights of a PhD 
Very interesting, thanks! If the clicking becomes too tedious (multiple or complex curves), you can alternatively use an automatic tool of mine, called Recurve – open-source, @ https://gen-img-dec.sourceforge.io/, https://github.com/svn2github/gid . It scans a picture and produces a .csv from it.
Thanks for that… such a tool would certainly be useful for plot with lots of points…