Graph plotting has rounding issue(s)

[EmmanuelCharpentier]

Is there an existing issue for this?

• [X] I have searched the existing issues for a bug report that matches the one I want to file, without success.

Did you read the documentation and troubleshoot guide?

• [X] I have read the documentation and troubleshoot guide

Environment

- **OS**: Debian testing running on core i7 + 16 GB RAM
- **Sage Version**: 10.1.beta5

Steps To Reproduce

Graph plotting logic seems to have issues with "marginally inexact" coordinates for vertex positions, best illustrated by an example :

sage: reset()
sage: set_random_seed(0)
sage: g=graphs.CubeGraph(2)
sage: gs=[u for u in g.connected_subgraph_iterator()]
sage: gs[11].get_pos()
{'01': (6.123233995736766e-17, 1.0), '00': (0.0, 0.0)}

[ The curious numerical values of the position coordinates are questionable, but not the subject of this issue ].

Expected Behavior

sage:  h=copy(gs[11])
sage:  h.set_pos({t:tuple(map(round, h.get_pos()[t])) for t in h.get_pos().keys()})
sage: h.get_pos()
{'01': (0.0, 1.0), '00': (0.0, 0.0)}

sage: h.plot()
Launched png viewer for Graphics object consisting of 4 graphics primitives

displays the subgraph (two vertices and an edge)

Actual Behavior

sage: gs[11].plot()
Launched png viewer for Graphics object consisting of 4 graphics primitives

displays two vertices without an edge :

Additional Information

Original finding : see ask.sagemath.org.

[dcoudert]

A solution is to stop using from math import sin, cos, pi and converting positions to floats. We can use instead from sage.functions.trig import sin, cos and from sage.symbolic.all import pi. The generation of the graph will be slightly slower but it should solve several issues (plot, curious numerical values, etc.).

This may however require to use the rational field for fractions, etc. See for instance graphs.GolombGraph?? in file src/sage/graphs/generators/basic.py. This is a significant amount of work but we might gain stability and avoid weird plotting issues.