Increase the number of connection points on pools
When connecting multiple message flows to a single pool, it's very difficult to have straight lines, even after you implemented the new auto-align feature.
The problem is that there isn't enough connection points on the
pools. When you stretch a pool over the length of a long diagram and
there are numerous message flows to link to a single pool, the space
between two connection point is too wide and make is almost impossible
to have straight lines without messing up with the diagram.
double or triple the number of connection points, or make the number of
connection points variable according to the pool's width so they aren't
too far from each other when the pool is stretched.