I like to know if you can have round arcs and straight arcs under one graph using igraph.
Yes; edge.curved=... may be a vector containing one item for each edge. TRUE means curved edge, FALSE means straight edge; you may also pass numbers to control how curved the edges should be.
Quoting the documentation:
"First the vector is replicated to have the same length as the
number of edges in the graph. Then it is interpreted for each edge
separately. A numeric value specifies the curvature of the edge;
zero curvature means straight edges, negative values means the
edge bends clockwise, positive values the opposite. TRUE
means curvature 0.5, FALSE means curvature zero."
Second, ggplot2 allows me to add lines to existing plot, can igraph provides this option?
Sorry, I don't know, I'm not that familiar with R.
All the best,
Tamas