Can't plot in Julia - plot

Question

I am currently reading a book (BRML) which has a demo (earthquake demo, exercise 1.22), which is written in Julia. I have never used Julia (although used Python and other languages quite extensively) before so I'm a complete noob.
What exactly does the line plot(x,y,".") do in the following code:
Pkg.add("Pkg")
using Pkg
Pkg.add("PyPlot")
S=5000 # number of points on the spiral
x=zeros(S); y=zeros(S)
for s=1:S
theta=50*2*pi*s/S; r=s/S
x[s]=r*cos(theta); y[s]=r*sin(theta)
end
plot(x,y,".")
I understand everything that is done before that, however I'm not sure what that specific line does. The reason I can't see for myself is because when I'm trying to run it on an online Julia compiler, I get the following error:
INFO: Initializing package repository /home/cg/root/4655378/.julia/v0.6
INFO: Cloning METADATA from https://github.com/JuliaLang/METADATA.jl
ERROR: LoadError: GitError(Code:ERROR, Class:Net, curl error: Could not resolve host: github.com
)
Stacktrace:
[1] macro expansion at ./libgit2/error.jl:99 [inlined]
[2] clone(::String, ::String, ::Base.LibGit2.CloneOptions) at ./libgit2/repository.jl:276
[3] #clone#100(::String, ::Bool, ::Ptr{Void}, ::Nullable{Base.LibGit2.AbstractCredentials}, ::Function, ::String, ::String) at ./libgit2/libgit2.jl:562
[4] (::Base.LibGit2.#kw##clone)(::Array{Any,1}, ::Base.LibGit2.#clone, ::String, ::String) at ./<missing>:0
[5] (::Base.Pkg.Dir.##8#10{String,String})() at ./pkg/dir.jl:55
[6] cd(::Base.Pkg.Dir.##8#10{String,String}, ::String) at ./file.jl:70
[7] init(::String, ::String) at ./pkg/dir.jl:53
[8] #cd#1(::Array{Any,1}, ::Function, ::Function, ::String, ::Vararg{String,N} where N) at ./pkg/dir.jl:28
[9] add(::String) at ./pkg/pkg.jl:117
while loading /home/cg/root/4655378/main.jl, in expression starting on line 1

Answer 1

As the third line indicates, the book is using the PyPlot package, which is basically a Julia wrapper around Python's pyplot.
So, we could refer to pyplot's documentation to figure out how that line of code works. But as mentioned in that page, pyplot is trying to emulate MATLAB's plot function, and for this case their help page is easier to navigate. As mentioned there,
plot(X,Y) creates a 2-D line plot of the data in Y versus the corresponding values in X.
and plot(X,Y,LineSpec) in addition "creates the plot using the specified line style, marker, and color." Clicking on LineSpec, we can see in the second table that '.' is one of the markers, with description Point and the resulting marker a black filled dot. So plot(x,y,".") creates a plot with dots as markers at the points specified by the x- and y-coordinates.
We could also try one of the other markers, for eg. plot(x,y,"+") creates this instead:
where if you look carefully, you can see that the points are marked by + signs instead.