Let me expatiate a little:

In my branch, oil gas and the like
at a certain stage during the project,
the engineer issues several orders or contracts
to several suppliers or contractors.
These are then compelled contractually
to deliver or to operate at dates or periods according to planning.

The cotton line allows the engineer to focus on those
tasks that endanger the sequential flow of work
and to correct the situation before a given contractor (an electrician) appears on
the site at the contractual date only to find out
that the electric substation is not yet ready for him...
If you "update" your planning with the delay on the
substation, then the electrician is not due for another month
and that would be misleading, because he is, "contractually".

I hear some of you utter the word "baseline",
but a simple gannt is a tough enough chart to read;
if you add the initial bars, nobody will be able
to decipher it among the construction or the direction teams...

james

you just seemed so dont know in love with trevors responses

ha ha ha

james

you sound like you fancy trevor, come on leonard were going

ha ha ha

Nicolas, fair call but you have put quotes around things that I didn’t actually say.
As far as I know, there is no way to change the appearance of the progress lines except via Tools, Tracking, Progress Lines, Line Styles, and changing the weight of the line just isn’t there.

It is a case of
a) I cannot give you what you want because as far as I know, and I am pretty sure I know about this but I could be wrong, it doesn’t exist.
b) I am pretty sure I know about this but I could be wrong.
c) in my opinion, progress lines serve very little purpose d) I suggest that you might like to consider the possibility that you dont need it, because it, along with other formatting "nice to haves" is not very important compared to other priorities such as getting the modeling and tracking right.
e) but you can choose to continue to need it and search for a solution, which might exist but see a) and d) above).

You said:

"The progress bar, thunder bar or cotton line
is not so useless if you leave your Gannt
the way you planned it initially
i.e. not as built."

... but this is never supposed to happen, since the tracking and updating is the whole point of doing the planning in the first place. You say it is not useless, but what is it useful for? At most, this would just be an intermediate step immediately before moving unused duration to to the right of the Status Date and progress in the future back to the left of the Status Date. The zig-zag line is slightly useful to indicate Tasks where this has to be done to iron out the zigs and zags, but once they are gone there is no further need for it, and its appearance for this limited purpose doesn’t matter.

Absolutely correct.
It’s always nice to find that I agree with someone.
Once upon a time in the newsgroup microsoft.public.project this came up and everyone who knows what they are doing says the same thing.
http://groups.google.com/group/microsoft.public.project/search?group=mic...
Rod Gill and Gerard said that Microsoft only put them in because someone in Japan wanted it, because it resembled doing Gantt Charts with coloured paper tape and elastic progress lines with pins.
The MSP help says that they measure progress. They don’t.
If the line zigs to the left, that means there is unused Duration that needs to be moved to the right of the Status Date (Re-schedule Work Button on the tracking Toolbar). If the line zags to the right, that means that there is duration elapsed in the future (of the Status Date), which is just plain impossible, and that should all be moved back to the left of the Status Date. When everything has been updated and re-scheduled there should be no zig-zag.

Progress bars suck and serve no useful purpose. better to not bother.