Drawing ellipsis between nodes in graphviz - graphviz

Question

I'm interested in drawing vertical ellipsis between nodes in graphviz like seen below:
The problem I'm having is whenever I try to do this I cannot seem to get x3 and xn to line up vertically as seen here:
Here is what I have tried:
digraph G {
rankdir=LR
splines=line
subgraph cluster_0 {
color=white;
node [style=solid, color=black, shape=circle];
x1 x2 x3 xn [group=g1];
label = "Input Features";
}
subgraph cluster_1 {
color=white;
node [style=solid, color=red2, shape=circle];
a1 [group=g2];
label = "Activation";
}
subgraph cluster_2 {
color=white;
node [style=solid, color=green, shape=circle];
out [group=g3];
label = "Output";
}
x1 -> a1;
x2 -> a1;
x3 -> a1;
a1 -> out;
x3 -> xn [arrowhead="none", color="black:invis:black"];
}
I'm very new to graphviz so I'm not even sure if I'm using subgraph properly here. I also tried adding the nodes in the subgraphs to groups, but that didn't seem to do anything.

Answer 1

Add
{ rank = same; x1 x2 x3 xn }
x1 -> x2 -> x3[ style = invis ];
to your first subgraph. This has the effect that
the four nodes are all one one level, i.e. lining up vertically
the three numbered nodes stay together
Here my version:
digraph G
{
rankdir = LR
splines = line
subgraph cluster_0
{
color = white;
node[ style = solid, color = black, shape = circle];
{ rank = same; x1 x2 x3 xn }
x1 -> x2 -> x3[ style = invis ];
label = "Input Features";
}
subgraph cluster_1
{
color = white;
node[ style = solid, color = red2, shape = circle ];
a1;
label = "Activation";
}
subgraph cluster_2
{
color =white;
node[ style = solid, color = green, shape = circle ];
out;
label = "Output";
}
x1 -> a1;
x2 -> a1;
x3 -> a1;
a1 -> out;
x3 -> xn[ arrowhead = "none", color = "black:invis:black" ];
}
which gives you
E D I T to answer the question in your comment; the key is reversing the order of node definitions and edge direction within the same rank, probably caused by the rankdir = LR layout. After all, there is a simple solution!
digraph G
{
rankdir = LR
splines = line
subgraph cluster_0
{
color = white;
label = "Input Features";
node[ style = solid, color = black, shape = circle ];
/* define and connect in reverse order */
{ rank = same; xn x3 x2 x1 }
x3 -> x2 -> x1[ style = invis ];
xn -> x3[ arrowhead = "none", color = "black:invis:black" ];
}
subgraph cluster_1
{
color = white;
node[ style = solid, color = red2, shape = circle ];
a1;
label = "Activation";
}
subgraph cluster_2
{
color =white;
node[ style = solid, color = green, shape = circle ];
out;
label = "Output";
}
{ x1 x2 x3 } -> a1;
a1 -> out;
}