The Timeline allows different scale groups to be displayed on the vertical axis, but the horizontal axis is always used to represent time. The XY Plot view allows you to view the plot with a quantity on the X axis instead.
Once a plot has been loaded, the XY Plot panel will display the plot with a quantity on the horizontal axis and a scale group on the vertical axis.
To change the quantity on the horizontal axis, use the drop-down box labeled “X Axis Quantity” underneath the plot.
Click on Window > XY Plot in the window menu to toggle the XY Plot panel’s visibility.