No problem with your table set up.
Header column will be ignored.
Header row labels will be used to label the chart.
Select the data block (X2:AA9 in the example below)
Click the Charts button and select the scatter chart.
You'll get a chart that doesn't look right.
Now click the 'gear' icon to the upper left of the selected data to show the menu below:
Choose "Share X values"
Your selection and chart should now change to resemble the one below:
Changes from default chart attributes:
In the Chart Inspector:
I selected Series, and chose to connect the data points with straight line segments, and reduced the size of the data point symbols to 8 points.
I selected Axis, and set the number of steps for each axis to give me labels that were all integers.
On the Table:
I clicked the colour square above each Y column then used the Stroke section of the format bar to increase the line weight to 3 points.
A line chart is a "Category" chart.
One axis, usually the 'x' axis, is a Category axis. The category axis does not show numerical values (even if the values are numbers, they are simply text that names each category). Categories are evenly spaced along this axis. Labels for the categories must be placed into a Header Column.
Values shown on the Value axis (usually the 'y' axis) are numerical.
Here are two examples of the same data, presented first as a scatter chart with the data points connected by line segnments, then as a line graph.
Note the differences in organization of the table, and the differences in spacing along the x axis.
All data in 'body' cells. Series labels in Header row. Note the shape of the line for Data series B (Green) and C (Yellow).
Category labels in a Header column (Column A), Data series labels in a Header Row (Row 1). The 'numbers ' in column A are seen as text, and a regularly spaced along the x axis.
Data series in regular cells (B2:D8) Only the data cells are selected when making the chart.
Note the shapes of the lines for data series B (Green) and C (Yellow).
For your setup, you would need to place the 'x' values into a Header column. The Y values in columns Y, Z and AA could stay in those columns.
You would select only columns Y, Z and AA, then make the Line chart, as I've done in the example below after inserting some new columns to the left of that data.