The X-axis, which is the horizontal axis, on most Excel charts does not use numeric intervals like the vertical Y-axis does. The X-axis contains either strings of text or a date under each set of data ...

How Excel Charts Work A chart has an x-axis range and a y-axis range to show the correlation between two different data sets. The x-axis is the horizontal category line.

If you want to add labels to the bubbles in an Excel bubble chart, you have to do it after you create the chart. Mary Ann Richardson explains what you need to do to add a data label to each bubble.