Write using mouse on R plot? - r

Question

I created scattergram using the plot() function in R.
Is there any possibility to draw on this graph?
I would like to add a straight line and get parameters of it, but in my opinion abline() can be inconvenient (I would like to draw many lines and choose one which will be most proper).
How can I accomplish this task?

Answer 1

Take a look at RStudio and this example:
library(manipulate)
data = matrix(rnorm(20), ncol = 2)
example <- function(data, a, b){
plot(data[,1],data[,2])
abline(a = a, b = b)
}
manipulate(
example(data, a, b),
a = slider(-5,5),
b = slider(-5,5)
)
This will put a new line on the plot, and allow you to tweak its slope and intercept.
This was inspired by the example on this page: http://support.rstudio.org/help/discussions/questions/106-rstudio-manipulate-command
Note that this requires installing RStudio (it ships with the manipulate package, I believe). For more info, see the site.
Others' solutions with locator can be done in base R.

Answer 2

Use locator(), a function that allows you to get the coordinates of the mouse pointer when clicking on a plot. Then use
plot(cars)
xy <- locator(n=2)
lines(xy, col="red", lwd=5)
lm(y~x, xy)
abline(coef(lm(y~x, xy)))
coef(lm(y~x, xy))
(Intercept) x
33.142094 1.529687
Of course the correct way of fitting lines through data is to use a proper model. Here is how you can do it with lm:
abline(coef(lm(dist~speed, cars)), col="blue")
I made the following graph with this code:
The thick red line is the line connecting my two mouse clicks
The black line is the abline through these points
The blue line is the line of best fit produced by lm
Warning 1: locator only works on some graphics devices. See ?locator for more details.
Warning 2: Drawing lines of fit by hand could well be a really stupid idea. Use a regression function like lm or a smoothing function like loess instead.

Answer 3

If you were hoping to add horizontal or vertical lines to your plot interactively, you may want to use the locator() function to capture the position of a mouse click on the plot.
For example, the following code would allow the repeated addition of vertical lines to an existing plot:
repeat {
click.loc <- locator(1)
if(!is.null(click.loc)) abline(v=click.loc$x)
else break
}
You could adapt this for horizontal lines with abline(h=click.loc$y)