draw side by side graphs and make them more clear

Question

I have two problems and sincerely I am trying to resolve them, but unfortunately I am not able...That is why I will appreciate very much your help.

1) My article architecture consists of two columns each page. That is why, in the beginning of my code, I wrote \begin{multicols}{2}. Now, and in a specific part of my article, I am interested to draw multiple graphs. Let us consider 4 graphs. I need that the first two graphs be drawn next to each other in the same line, then the two last must be drawn below the previous graphs and next to each other in the same line. To make that, firstly, I wrote \end{multicols}, but although that, when I draw my 4 graphs, they are drawn each one below the other...

2) My second problem is that in each graph, they are a huge number of points, that is why, when the graph is drawn, it is not being clear. In other words, all the points being very small and the user needs to zoom in the article in order to show clearly the points. So how can I make the graph more clear and visible.

Let us consider my graphs that contains in this example a few points (they contains a huge number of points in my article. Here is just a simple example):

    \documentclass{article}
    \usepackage{pgfplots}
    \begin{document}


    \begin{tikzpicture}
    \begin{axis}[
        axis lines=middle,
        xmax=20,
        ymax=20,
        xtick=\empty, ytick=\empty
]
    \addplot [only marks] table {
    -10 -4
    -8  2
    -5  5   
};
    \end{axis}
    \end{tikzpicture}



    \begin{tikzpicture}
    \begin{axis}[
        axis lines=middle,
        xmax=20,
        ymax=20,
        xtick=\empty, ytick=\empty
]
    \addplot [only marks] table {
    -12 -4
    -5  2
    -5  4   
};
    \end{axis}
    \end{tikzpicture}



\begin{tikzpicture}
    \begin{axis}[
        axis lines=middle,
        xmax=20,
        ymax=20,
        xtick=\empty, ytick=\empty
]
    \addplot [only marks] table {
    -13 -4
    -1  2
    -8  4   
};
    \end{axis}
    \end{tikzpicture}




\begin{tikzpicture}
    \begin{axis}[
        axis lines=middle,
        xmax=20,
        ymax=20,
        xtick=\empty, ytick=\empty
]
    \addplot [only marks] table {
    -12 -4
    -1  11
    -8  4   
};
    \end{axis}
    \end{tikzpicture}


    \end{document}

Answer 1

Here is a solution based on groupplots and figure*:

1. first 2x2 figures are fit within a single column:

2. and the next 2x2 group spans over the page:

MWE:

\documentclass[twocolumn, a4paper, 10pt]{article}
\usepackage{pgfplots}
\usepgfplotslibrary{groupplots}

\usepackage{lipsum}

\begin{document}
\begin{figure}[htbp]
\centering
\begin{tikzpicture}
\begin{groupplot}[group style={group size=2 by 2}, width=0.25\textwidth]
    \nextgroupplot[
    axis lines=middle,
    xmax=20,
    ymax=20,
    xtick=\empty, ytick=\empty]
    \addplot [only marks] table {
        -10 -4
        -8  2
        -5  5  
    };
    \nextgroupplot[
    axis lines=middle,
    xmax=20,
    ymax=20,
    xtick=\empty, ytick=\empty]
    \addplot [only marks] table {
        -3 -4
        -5  2
        -12  5
    };
    \nextgroupplot[
    axis lines=middle,
    xmax=20,
    ymax=20,
    xtick=\empty, ytick=\empty]
    \addplot [only marks] table {
        -7 -15
        -8  3
        -12  5
    };
    \nextgroupplot[
    axis lines=middle,
    xmax=20,
    ymax=20,
    xtick=\empty, ytick=\empty]
    \addplot [only marks] table {
        -8 -2
        -6  1
        -12  5
    };
\end{groupplot}
\end{tikzpicture}
\caption{My figure.}
\end{figure}

\begin{figure*}[htbp]
\centering
\begin{tikzpicture}
\begin{groupplot}[group style={group size=2 by 2}, width=0.4\textwidth]
    \nextgroupplot[
    axis lines=middle,
    xmax=20,
    ymax=20,
    xtick=\empty, ytick=\empty]
    \addplot [only marks] table {
        -10 -4
        -8  2
        -5  5  
    };
    \nextgroupplot[
    axis lines=middle,
    xmax=20,
    ymax=20,
    xtick=\empty, ytick=\empty]
    \addplot [only marks] table {
        -3 -4
        -5  2
        -12  5
    };
    \nextgroupplot[
    axis lines=middle,
    xmax=20,
    ymax=20,
    xtick=\empty, ytick=\empty]
    \addplot [only marks] table {
        -7 -15
        -8  3
        -12  5
    };
    \nextgroupplot[
    axis lines=middle,
    xmax=20,
    ymax=20,
    xtick=\empty, ytick=\empty]
    \addplot [only marks] table {
        -8 -2
        -6  1
        -12  5
    };
\end{groupplot}
\end{tikzpicture}
\caption{My figure.}
\end{figure*}

\lipsum[2-10]
\end{document}