Save an image of the scatterplot as a PNG, JPG, or BMP file.
Load the digitize package with library(digitize). [If you have not done so previously, you may need to install the digitize package by selecting the Packages tab in the lower-right pane of RStudio, then the Install button, then type digitize in the ensuing dialog box, and press Install.]
Set the working directory to where the image file is located with setwd().
Run dat <- digitize("filename") with filename replaced by the name of the image file (including the extension).
On the image in the plot window …
Click on an obvious “lower” point on the x-axis.
Click on an obvious “higher” point on the x-axis.
Click on an obvious “lower” point on the y-axis.
Clikc on an obvious “higher” point on the y-axis.
You will then be prompted to enter the actual values (read these off the plot) for each of the four points that you clicked on. These are called “return values.”
Then carefully click on each point on the plot.
After you have clicked on the last point either press the “Finish” button on the plot window or press the “ESC”ape key. The data are then saved into an object called dat.
Write the data in dat out to a file with write.csv(dat, "filename.csv", quote=FALSE, row.names=FALSE) where filename is replaced with a name for your data file.
In the future, you can then load these data into R in the usual way (i.e., using read.csv()).