Apologies if this is a complete newbie question, but as others have noted, I find the OmniGraffle doc/Help a little lacking in detail in places.

Is it possible to attach a magnet (or its equivalent) to a line, as well as to a shape? I do a lot of flowcharting, and often need to have flows from multiple places join a "main line" along its length, so they all converge at a single target via the single main line (making any sense?). Ideally I would like to attach magnets along the main line as endpoints for the other lines, so that as the whole thing get moved around all the attachments are retained.

[JKT]

There are two ways to do this - the results do differ by the way the attachment point behaves when you move the ends of the target line.

Method 1:

A. Create your first line.
B. Create a second line and drag the end to attach it to the first line. A magnet will be created at the point you connect the line. You can drag the magnet anywhere to position it.
C. When you move the end points of the first line, the entire line will move and drag the attached lines with it (important difference relative to method below)

Method 2:

A. Depending on your Preference setting for Line Label Creation in General preferences, you will either double or you will option-double click a line to add an extra point (=magnet) anywhere on the line's length.
B. Attach any secondary lines to this magnet.
C. Drag the ends of the first line and this time, only the end of the line will be moved. Your additional point(s) and the attached lines will stay where they were.