I missed what was staring me in the eye - the values in the second column are left-aligned. This suggests that they are text values instead of "real" numbers, so Excel treats them as 0.
Do the following:
- Select the values in the second column.
- Set the number format to Number, with 1 decimal place.
- Select an empty cell.
- Copy it.
- Select the values in the second column again,
- Click the lower part of the Paste button on the Home tab of the ribbon.
- Select Paste Special...
- Select Add.
- Click OK.
- The values should now be right-aligned, and the chart should look as intended.