NavList:

That anonymous person was me.  I hit "send" before entering name etc (forgetting this necessity under the new system).

On the chart I think I see two lines, but will go with the northern-most which appears to be close to Porto.

One point of interest: there is a prominent mark on the chart which is well off the coast.  This appears to be the starting point for the track - which is itself suspiciously straight, suggesting an intended track rather than actual.

This is consistent with how I learned and probably many others here on navlist - at least amongst the maritme rather than aviation sub-groups.  The prominent mark is the Departure Point ("DP") and marks the transition from pilotage to celestial nav.  If not racing, I would take a series of compass bearing off the highest land available and would plan my DP to get the best "cross" (angle between bearings) if I had two or more good landmarks for bearings.  In this way I had a fix which was already many miles off the coast and thus reduced the distance traveled between a reliably known position and the first celestial position line.

As for the straight track, it was my practice to mark the intended track on the chart, but to mark the subsequent positions (as dots/triangles/crosses) and not to draw in the actual track (which in any even was never straight).  This not only reduced marks on the chart (as the rest of the work was done on a plotting sheet) but made an easy and understandable reference for a curious crew. 

Final point.  The straight line goes out only about 400-500 miles (I didn't analyze the distance against the chart). Why does it end?  There are two options here.

If this is a trans-Atlantic trip, Cap'n Onedin is going to sail a simulation of a great circle route via a series of short rumblines (subject to his weather plan and the previaling winds).  He has planned his first way-point being a rumbline route over a distance short enough so as not to add much distance over a theoretical great circle.  He will remeasure based on celestial fixes and adjust accordingly.  So there is no point is computing a planned track beyond a distance where he will recompute his new great circle waypoint. 

He is taking a northern route which suggests the hurricane season (avoid the soutern route) and the ability to sail to windward.  Those who remember the show will know that although the opening scenes feature a beautiful square rigger, he owned a ship with a fore and aft rig that would have been reasonably close winded.

Alternatively, if he is sailing back to, say, Liverpool, he is planning to sail north west as much as he can fetch against headwinds until he picks up the westerlies. They could kick in at any time, so there is no point in planning a track further than where he hopes to get the wind change.

Captain Onedin knew his stuff.