Trendlines: When They Will Be Generated, And When They Will Not BeFor A Scatter Chart

Hello,

I posted the following to a general MS Excel Group but did not receive
any replies. Then, I found out that that there is a specific Charting
Group for Excel users, so will transfer the message here.

If anyone could help me, would really appreciate it.

Have been going around circles with this, trying to understand.
Using Excel 2007.

If anyone could explain for me the limitations of when a Trendline
(linear) will, or will not, be drawn on a Scatter chart I would be most
appreciative.

My data looks like this:
Col A Col B
Jan 1 5
Jan 2 6
Jan 3
Jan 4 4
Jan 5 8
etc.

Many of the dates have no data in Col. B (as shown for Jan 3, e.g.)

If I:

go to:

Select Data
Hidden And Empty Cells
"Show Empty Cells As Gaps" Box Is Checked

Then, The Trendline Shows Up O.K.

But, if I check the box for: Connect Data Points With Line, the
Trendline then disappears.
And, will not allow being generated if this box remains checked.

Why ?

Recently, I had to install a new HD in my Desktop, as old one conked
out. Also re-installed Excel (2007) from MS disks. I am 99.9% certain
that before the HD and Excel re-installs,it did not act this way.

I remember that I always had lines drawn between all the actual data
points,and
Excel would merely skip dates for which I had no data; trendlines
would still appear and be drawn; no problem.

But, I admit, I might be wrong, although I don't think so. But, at my
age,...!

Again, I'm using Excel 2007, and scatter charts.

Any thoughts on ?

Thanks for help,
Bob