Two value-based axis for chart in Numbers 3.5
Took me a while, but I figured it out: The solution is to choose a scatter plot instead of a line chart.


Select Insert -> Chart -> 2D Scatter
Select the chart, then select Add Chart Data:

Now simply select your entire table, including the x axis. Numbers will figure out the rest and automatically select your leftmost column as the x-axis.

Finally, to make it look like a line chart, select the chart, then select the "Axis" tab in the inspector Chose Data Symbols: None and Connection Lines: Straight.


Result: