How to control the connection between edge and vertex?

In the graph below, the curved edge between 6 and 8 does not point to the centre of the vertex.
This is in contrast to the straight edge 5-6.
Is there any way to control the meeting point between edge and vertex?

require(igraph)
g <- make_graph(~ 1-2-5-3-4-5-6-7-8-1, 1-7, 2-4, 2-8, 3-6, 5-6, 5-7, 6-8, 8-3)
g$main <- "Meeting point between edge and vertex (edge6-8)\nHow to control?"

E(g)$curved <- 0
g <- set_edge_attr(g, name="curved", E(g)[15], -2)

q3 <- sqrt(3)
lyt <- c( 0, 1, 1.25, 1.50 , 2, 1, .50, .75 
        , 0, 0, q3/4, 1    , 0, 2, 1  , q3/4
        ) 
dim(lyt) <- c(8, 2)

plot(g, layout=lyt, edge.width=2, vertex.size=10, sub="Fig.1")