CODEWITHSUNDEEP
rplotsequenceline-plot
solanum
solanum

Reputation: 23

Why do I see points on the line when doing a line plot in base R?

When I plot a sequence in a line plot in R using transparent color settings I keep getting these dots on the line. Do you have an idea why this is and how I could solve the problem?

s <-seq(1,100, by = 0.1)
plot(s, type="l",col=scales::alpha("black",0.3), lwd=3)

Line plot in base R

I tried using the lines function which resulted in the same problem.

Also, it is interesting to see how the the dots change with e.g. s <-seq(1,100, by = 0.5)

and disappear with s <-seq(1,100, by = 1)

sessionInfo()
R version 4.1.2 (2021-11-01)
Platform: x86_64-apple-darwin17.0 (64-bit)
Running under: macOS 14.3.1

Thank you for your answers.

Upvotes: 2

Views: 193

Answers (3)

MrGumble
MrGumble

Reputation: 5776

It is the line end style that is at play here.

The "line"-plot is drawn as a series of individual lines:

(0,0)    -> (1, 0.1)
(1, 0.1) -> (2, 0.2)
(2, 0.2) -> (3, 0.3)
...

At each end of the line, the default is to draw a "rounded line cap". You can see this at each extreme of the line. Two overlapping rounded caps, give the illusion of a small circle (or point).

These points are visible, because of your partial transparency.

A solution could be to try the two other options "butt" or "square":

plot(s, type="l",col=scales::alpha("black",0.3), lwd=3, lend=1)
plot(s, type="l",col=scales::alpha("black",0.3), lwd=3, lend=2)

See the lend parameter in ?par.

Upvotes: 4

jay.sf
jay.sf

Reputation: 73562

You can set value v=0 in rainbow. No need for scales::alpha package or customizing a function (if it's just black).

> plot(s, type="l", col=rainbow(n=1, v=0, alpha=.3), lwd=3)

enter image description here

Note: I am on Linux.

Data:

s <- seq(1, 100, by=0.1)

Upvotes: 0

user2554330
user2554330

Reputation: 44957

This looks like a bug in some graphics drivers, but not others. In MacOS I see it if I use quartz(), pdf(), png() or jpeg() devices. I don't see it in X11(), the default device in RStudio, or the devices in the ragg package.

So the solution is to use a device that doesn't have the bug.

Upvotes: 4

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