Grid version of FR layout does not work properly with disconnected graphs

[Vini2]

Describe the bug
The layout produced from layout_fruchterman_reingold() looks disordered when plotting large graphs with thousands of vertices.

To reproduce

Run the following code:

from igraph import *

# Create graph
my_graph = Graph()

# Create list of edges
edge_list = [(10,3), (10,444), (8,4), (3,5), (5,4)]

# Add vertices
my_graph.add_vertices(2000)

# Add edges to the graph
my_graph.add_edges(edge_list)

# Set visual styles
visual_style = {}
visual_style["bbox"] = (3000,3000)
visual_style["margin"] = 30
visual_style["vertex_size"] = 30
visual_style["vertex_label_size"] = 12
visual_style["edge_curved"] = False

# Set the layout
visual_style["layout"] = my_graph.layout_fruchterman_reingold()

# Plot the graph
plot(my_graph, "testing.png", **visual_style)

Output graph testing.png:

I am not sure where the issue is occurring or starting from which version.

Version information
I came across this issue with versions 0.9.10, 0.9.9 and 0.9.8 from conda.

Versions that worked correctly
When I plot using python-igraph version 0.7.1.post7 from conda, I get the proper layout as shown below:

[szhorvat]

Use the grid=False option to the layout.

[szhorvat]

This happens because the grid version does not support disconnected graphs at all. Trying to add support for them in a naive way, by mirroring the non-grid version, fails. The failure most likely has to do with the fact that the vertex positions "drift" in the disconnected version for reasons that are unclear to me.

[szhorvat]

I made a fix that improves this, see f88495c At least this will certainly make it into the next igraph version (likely a bugfix version in the 0.9 series). We'll look into more improvements later, but no promises about including them in the 0.9 series.

With 1000 iterations and 2000 isolated vertices, now I get the following:

With grid:

Without grid:

A union of a regular tree and many isolated vertices, without and with grid:

[Vini2]

Use the grid=False option to the layout.

Thank you for the fix. For now I will use this option.

[szhorvat]

I made some additional improvements here: cbd51ae

I check the FR paper and they state that the grid version does not need any special treatment of disconnected graphs. This, with the above fix, I am closing this.