# https://gist.github.com/thigm85/5760134
digraph G {
    rankdir=LR
	splines=line
    nodesep=.05;
    bgcolor="transparent";
    penwidth = 0  
          
    node [label=""];
        
    subgraph cluster_0 {
		color=white;
        node [style=solid,color=blue4, shape=circle];
		x1 x2 x3 x4 x5 x6;
		label = "input layer";
	}

	subgraph cluster_1 {
		color=white;
		node [style=solid,color=red2, shape=circle];
		a12 a22 a32 a42;
		label = "hidden layer";
	}

	subgraph cluster_2 {
		color=white;
		node [style=solid,color=seagreen2, shape=circle];
		O1;
		label="output layer";
	}

    {x1; x2; x3; x4; x5; x6} -> {a12;a22;a32;a42};
    {a12; a22; a32; a42} -> O1
}