Another approach if you wish to dynamically choose where the phase occurs. I also will use the Control Chart Builder and the Global Data Filter.
Create a variable called PHASE that has the formula: If( Selected(), 1, 2 ).
Now go to the Control Chart Builder.
Drag Ib to the Y zone. Drag your Relative Time to the X-axis. Drag PHASE into the phase area at the top of the chart.
Now choose Rows > Data Filter. Specify your Relative Time as the filter. Drag the triangle for the upper end of Relative Time to various locations and watch the control charts get adjusted for the various "cut points".
You could also use Graph Builder with the Overlay as Thierry_S mentioned instead of the Control Chart Builder.