Problem in the `geom_segment`

[sammo3182]

@fsolt, I'm doing the final check for the vignette before releasing v0.2.5 and here's a problem I cannot solve. The last plot of the vignette was messed up when I rebuilt the vignette as following:

When I I went through the small_multiple function, everything looked fine until the function calling geom_segment at line 171. However, the setting of geom_segment looks about right. I can't find what causes the problem.
Do you get the same plot when building the vignette?