If your diagrams are not crazy full you should not need an extra-fancy algorithm for this, but just use some common sense.
Cover the surface with a rectangular grid and then find a way to connect to boxes with straight lines along grid lines with a minimum number of angles: if the boxes are not on the same grid lines and you don't care where you connect you need one angle if there is no other node in between. If there are e.g. nodes in the way you need at least one more angle.
As a second step for fuller diagrams add code that not only optimizes for minimal number of edges but also for minimal length of the lines. If your diagrams are not crazy full this should be hardly noticeable in terms of application response.
For extra eye candy round the angles taking into account the lengths of both legs and checking for intersections with other objects on the surface. I would use 90°-pies of circles and adjust the radius of the circles (apparently not what was done above) -- for longer legs the radius should be bigger. Maybe the toolkit you are using can help you here.
