Scatter chart

When you selected the X and Y data for the line chart it saw it as two series, not as X and Y. Line charts should have the X axis categories in a header column, that is what Numbers prefers. When creating a line chart you only select the Y data and it uses the values in the header as the X axis. Scatter charts, on the other hand, want X and Y to NOT be in header columns and you select both to make the chart.

If you are okay with converting the first column to a header column:

1. If the dates are in column A, convert it to a header column.
2. Select the Y values in column B and make your chart.

If you do not want to convert A to a header column:

1. Select only the Y values and make your chart.
2. Click on the chart and go to the formatting sidebar, to the Axis tab and then to the X axis
3. Where it says "label references", delete anything that is there then go to your table and select the cells that have your X axis data. This will make them the "label references" on the X axis.

Perhaps you could use a 2D Line Chart instead of a 2D Scatter Chart:

You can remove connection lines and do other stying by clicking the series in the chart and going to the Style tab:

SG