It can be done with a graphics script. Right click on the FrameBox and select Customize…
Or do it with jsl. For example:
dt = Open("$SAMPLE_DATA/Big Class.jmp");
Column("age") << set modeling type(continuous);
gb = dt << Graph Builder(Variables(X(:age), Y(:weight)), Elements(Points(X, Y, Legend(3))));
Report(gb)[Framebox(1)] << Add Graphics Script(
Description("reflines"),
bp = 13.5; //breakpoint
y1 = Mean(:weight[dt << get rows where(:age <= bp)]);
y2 = Mean(:weight[dt << get rows where(:age > bp)]);
Line(
Pen Size(2);
Pen Color("green");
{bp, Y Origin()};,
{bp, Y Range() + Y Origin()}
);
Line(
Pen Size(2);
Pen Color("blue");
{X Origin(), y1};,
{bp, y1}
);
Line(
Line Style(2);
Pen Size(2);
Pen Color("gray");
{bp, y1};,
{X Range() + X Origin(), y1}
);
Line(
Pen Size(2);
Pen Color("red");
{bp, y2};,
{X Range() + X Origin(), y2}
);
Line(
Line Style(2);
Pen Size(2);
Pen Color("gray");
{X Origin(), y2};,
{bp, y2}
);
);

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