Scatter chart x axis labels not changing to appropriate values

I have made a scatter plot chart with three different series to plot, all with the same intended x axis values. Under data selection, I did specify a range for the data that should appear as the x axis category labels, but it will not change to those values (14 days, 21 days, etc...), and instead display 1, 2, 3, etc., on that axis. Format Selection when selecting those x axis labels also doesn't give me the option to use text. Screencaps attached, showing formatting box options I see and the ranges for data selection, note that in that box it DOES show the correct x axis values, though the chart does not... Using 2007.

It can be hard to tell from a picture, but it looks like you have included the text string "days" in each value for the X values. This converts the entire cell to a text string, which Excel does not know how to use in an XY scatter plot. So Excel uses the count numbers (1,2,3,...) for the plotted X values.

Is it necessary to include the "days" string in the X values? The way I do it is to just use the numbers (14, 21, 28, ...) for the X values.