aha, ok, so the line is just placed in the general direction.
There are a few tiny things that could use better clarification, simple tiny things like making the 'X's larger that signify the variable for kilometers. at step 14.4 I had to wonder for a second what the X was, until I zoomed in and saw the tiny x on the plot! The info is there, its just sometimes a bit hard to find, and maybe someone with bad eyes wouldnt even see it! Could either make the X's a bit larger, or explain on the bottom of step 14 that X is the distance the ship has traveled in the time interval one is using.
Overall, great guide!
edit:
for convoys, one could simply manually listen for the median of the convoy and base off that knowing a medium convoy is about 5k wide and 3k forward, and so on. Yes?