If the first column values are not uniformly spaced, you should use an XY
chart, not a line chart. An XY chart automatically uses the first column for
X values, unless (as I discovered yesterday) the X and Y values all consist
of dates.
To ensure that Excel correctly parses your data for any chart type, put the
X values in the first column, and Y values in subsequent columns. Leave a
blank cell above the X values, and enter labels in the cells above each
column of Y values. The blank cell signals Excel not to guess, and that the
first column and first row are different from the rest of the data.
- Jon