It isn't clear from your question what data you are plotting, or the type of chart you are creating.
In the parentheses you have (1, 3.5), (2, 2.9), and (3, 2.7). If you have those number pairs in, say, columns A and B and you select them for the chart, then you will get 1, 2, and 3 on the X axis if you are creating an XY Scatter chart. Excel automatically puts the furthest left data as the X variable.
If you are creating a Line chart, then there really is no x variable and Excel is just labeling the X axis with their data point numbers. You can replace the X-axis labels in a line chart very easily. Just right click in the chart area and choose Source Data. Then, in the dialog box, go to the Series tab. At the very bottom you can specify a range of cells to be used for the X-axis labels.
Since you have numeric data and the X and Y variables are probably related, you should most likely be using an XY Scatter chart instead of a Line chart. So, enter your data into two or more columns and make sure that you X variable is in the first column.
Tim
http://www.tvmcalcs.com/blog/