Question

p = [3,3]
plot(p, 'x')

This weirdly generates this:

I’d like it to be a point at x=3/y=3 on the plot. How?

Answer 1

@mathematician1975 is right, but I feel like this requires a bit more explanation:

Like the official documentation states:

plot(Y) plots the columns of Y versus the index of each value when Y is a real number.

so in fact this is not weird at all that plot(p, 'x') plots each value in p against its index, i.e. the points (1, 3) and (2, 3).

This is actually handy in some cases (when you want the x-coordiantes to be a running index), but not in yours. To plot point p correctly, use the syntax plot(X, Y), that is:

plot(p(2), p(1), 'x')

(here I assumed that the y-coordinate is the first in p, but if it’s the x-coordinates you can just swap the places of the input arguments).

In the general case, if p is a matrix with two columns (say, the first contains all y-coordinates and the second all x-coordinates), you can plot all points like so:

plot(p(:, 2), p(:, 1), 'x')