Question

When I run this line:

plot(z$prod,z$x, type="l")

It plots the values as dashes and ignores the type command.
I can easily get it to plot the line if I run:

points(z$prod, z$x, type="l")

What is it about plot() that ignores the type command while points() recognizes it?

• z$prod is a factor with 4 levels
• z$x is a num[1:6]

This is driving me crazy!

Answer 1

Expanding on my comment…

The second paragraph you read when looking up ?plot (you did read ?plot right?) is this:

For simple scatter plots, plot.default will be used. However, there
are plot methods for many R objects, including functions, data.frames,
density objects, etc. Use methods(plot) and the documentation for
these.

Then continue reading about the type argument:

type: what type of plot should be drawn […] Note that some methods, e.g. plot.factor, do not accept this

---

Typing methods(plot) lists a whole bunch of functions. You note that your x values are a factor. Hmmm. There’s a plot.factor listed in there. I wonder what it does. Turning next to ?plot.factor we see that:

…For numeric y a boxplot is used…

And further,

Further arguments to barplot, boxplot, spineplot or plot as
appropriate. All of these accept graphical parameters (see par) and
annotation arguments passed to title and axes = FALSE. None accept
type.

There. Mystery solved.