When you first create a chart in Excel 2013, the result might appear confusing -- with some chart types, most notably line charts, Excel displays the X axis as "1, 2, 3..." regardless of the data you actually want as your X axis. For charts where you want to specify an X-axis range of numbers, you first need to switch to a scatter chart and then select the range.
Select your chart and pick a scatter chart style from the Insert tab to change the chart type.
Open the "Design" tab and press "Select Data." If you have two adjacent columns of data -- X-axis data on the left, Y-axis on the right -- select them and press "OK" to remake the chart using your X-axis range, and skip the following step.
Alternatively, if the X-axis data you want to use isn't situated directly to the left of the Y-axis data, press "Edit" in the Legend Entries section.
Select the X and Y ranges separately using the Series X Values and Series Y Values fields. Press "OK" on both dialog boxes to remake the chart.
Press "Add Chart Element" on the Design tab to add "Axis Titles" for each axis, clarifying their relationship.
Type in an axis title's name by hand, or select it, type "=" in the Formula Bar and click a cell that bears the axis's name.