Question

The following works, giving 3 points connected with a line.

plot(c(1,7,12), c(0,0,2),type="b")

However this does not work (it plots the points but without connecting line and without any warning or error)

t<-data.frame(x=1:20,y=c(0,NA, NA, NA, NA, NA,  0, NA, NA, NA, NA,  2, NA, NA, NA, NA, NA, NA, NA, NA))
plot(t$x, t$y,type="b")

Why is that? Is it because of the NA in the data frame? I can’t seem to find any reference on this.

Answer 1

That is the intended, documented, and (when you need to plot lines with breaks in them) desirable behavior of R’s lines() function. From the ?lines help page:

The coordinates can contain ‘NA’ values. If a point contains ‘NA’
in either its ‘x’ or ‘y’ value, it is omitted from the plot, and
lines are not drawn to or from such points. Thus missing values
can be used to achieve breaks in lines.

To get lines between the points, simply remove the points for which y has a value of NA:

d <- data.frame(x=1:20, 
              y=c(0,NA,NA,NA,NA,NA,0,NA,NA,NA,NA,2,NA,NA,NA,NA,NA,NA,NA,NA))

plot(y ~ x, data = d, type = "b", subset = !is.na(y))