`start_cap` ignored in some cases if the graph contains self-edges?

[ha0ye]

I noticed this while trying to add spacing between the nodes and edges in my figure and traced it back to whether there were self-edges (e.g. A -> A) in the graph.

For instance, contrast the output of

data.frame(from = c("a", "b"), 
                to = c("a", "c")) %>%
    graph_from_data_frame() %>%
    ggraph(layout = "circle") + 
    geom_edge_fan(start_cap = circle(1), 
                  end_cap = circle(1), arrow = arrow()) + 
    geom_node_circle(aes(r = 0.1)) + 
    geom_node_text(aes(label = name)) + 
    coord_fixed()

to

data.frame(from = c("a", "b"), 
                to = c("b", "c")) %>%
    graph_from_data_frame() %>%
    ggraph(layout = "circle") + 
    geom_edge_fan(start_cap = circle(1), 
                  end_cap = circle(1), arrow = arrow()) + 
    geom_node_circle(aes(r = 0.1)) + 
    geom_node_text(aes(label = name)) + 
    coord_fixed()

[malcolmbarrett]

Interestingly, this issue seems not to apply to geom_edge_loop() but does on other edge geoms (I've tried geom_edge_link() and geom_edge_arc())

data.frame(from = c("a", "b"), 
                to = c("a", "c")) %>%
    igraph::graph_from_data_frame() %>%
    ggraph(layout = "circle") + 
    geom_edge_link(start_cap = circle(1), 
                  end_cap = circle(1), arrow = arrow()) +     
    geom_edge_loop(start_cap = circle(1), 
                  end_cap = circle(1), arrow = arrow()) + 
    geom_node_circle(aes(r = 0.1)) + 
    geom_node_text(aes(label = name)) + 
    coord_fixed()

Created on 2018-08-03 by the reprex
package (v0.2.0).