I encounter this problem both in Excel 2003 and Excel 2007:

I created a simple scatter chart with lines, with only two datapoints:

x y
0 0
1 1

This gives a correct chart with a straight line through {0,0} and {1,1}.

The problem occurs when I change the axis minimum of both axes to 0.999.

Excel should display a chart with a straight line through {0.999,0.999} and
{1,1}. But it does not. The line is crossing the x-axis somewhere around
0.9994 (different exact value in both Excel versions).

Can I solve this problem with changing Excel settings or would this be
caused by a bug in the software?