You've not said which version of Excel you're using, but it should be similar for most recent versions I think:
- Choose Insert -> Charts -> Insert Line Chart -> Line from the ribbon to insert a blank line graph.
- With the graph selected, click the Select Data option from the ribbon
- With the cursor in the Chart data range text box, highlight the cells containing your Y values
- On the right-hand side, under Horizontal (Category) Axis Labels, click Edit
- Select the range containing your X axis values and press OK
- Press OK to close the chart editor dialog.