If you're saying that bar charts don't work in this case (ie the bars
are too thin to be of any use) then the software should prevent people
from specifying bar charts under these conditions.

Your "advice" to changing the gap is a non-starter....it doesn't work.
Its beginning to sound like this is a lost cause...

Tushar Mehta wrote:
What do you expect XL to do? You want sufficient horizontal space for
365 entries and XL is giving it to you. The result is that each day
occupies only 1/365th of the horizontal space of the plot area.

You *might* get some additional thickness by double-clicking the
plotted series, selecting the Options tab, and setting the Gap Width to
zero.