Sine function in pgfplots and MATLAB. Why are different results obtained?

Question

I did the graph (view image) of the absolute value of sine function in pgfplots and MATLAB. Why are different results obtained?

pgfplots:

\begin{tikzpicture}[scale=1.2]

    \begin{axis}[
    domain = 0:2*pi,
    yticklabels={-2,-1,0,1,2},
    axis lines = middle,
    axis on top = true,
    grid = major,
    grid style = {dashed, line width=.1pt, draw=gray!30, /pgfplots/on layer=axis background},

    ]
    
    \addplot+[no marks, color=blue] {abs(sin(deg(x)))};

    \end{axis}
\end{tikzpicture}

MATLAB:

x = 0:0.01:2*pi;
y = sin(x);
z = abs(y);

plot(x,z,...
    'LineWidth',1,...
    'Color','blue');

Answer 1

You need to use more samples to make sure that pi is one of the x-values used to calculate the y-values.

The number of samples can be set using an optional argument to the \begin{axis} command.

\begin{axis}[samples=1000,...]

Answer 2

There are a couple of problems here.

First off, the number of sample is too small by default for your case --- you should ask for more, and, if you want to get the value in the middle of the domain, use an odd number of them.

samples = 101, 

Then, you use xticklabels which puts the given labels at the (automatically calculated) ticks position... you should almost never use xticklabels without an explicit xtick. In this case, they are not needed.

Additionally (but this is a matter of style) I think that the graph is better with a bit of breathing, and indicating the coordinates (0,0); both effects are achieved removing axis lines = middle. So:

\documentclass[border=10pt]{standalone}
\usepackage{tikz}
\usepackage{pgfplots}\pgfplotsset{compat=1.13}
\usetikzlibrary{arrows.meta,positioning,calc}
\begin{document}
\begin{tikzpicture}[]
    \begin{axis}[
    domain = 0:2*pi,
    samples = 101,
    axis on top = true,
    grid = major,
    grid style = {dashed, line width=.1pt, draw=gray!30, 
    /pgfplots/on layer=axis background},
]
\addplot+[no marks, color=blue] {abs(sin(deg(x)))};
\end{axis}
\end{tikzpicture}
\end{document}

...and to show the xticklabels thing, if you add:

xtick = {0, 1.5708, ..., 7},
xticklabels = {$0\mathstrut$, $\pi/2$, $\pi\mathstrut$, $3\pi/2$, $2\pi\mathstrut$},

you have: