Thanks Experts!
I tried to write like this and also asked the AI, but still failed.
I knew before that this could make the picture lighter, but it's not transparent.
Craige specifically put forward a suggestion for achieving background transparency (in JMP 17, but JMP18 did not improve).
dt = Open("$SAMPLE_DATA/Big Class.jmp");
p1 = dt << Graph Builder(
Show Control Panel(0), Show Legend(0), Show Title(0), Show Footer(0), Show X Axis(0), Show Y Axis(0), Show X Axis Title(0), Show Y Axis Title(0),
Variables(X(:height), Y(:weight), Overlay(:sex)),
Elements(Points(X, Y, Legend(1)), Smoother(X, Y, Legend(2)))
);
img = p1 << Get Picture;
m = img << Get Pixels("rgba");
size_info = img << Get Size;
img_width = If(N Items(size_info) >= 1, size_info[1], 0);
img_height = If(N Items(size_info) >= 2, size_info[2], 0);
n = m;
white_pixels = Loc(m[0, 0, 1] == 255 & m[0, 0, 2] == 255 & m[0, 0, 3] == 255);
for(k = 1, k <= N Rows(white_pixels), k++,
i = white_pixels[k, 1];
j = white_pixels[k, 2];
n[i, j, 4] = 0
);
img << Set Pixels("rgba", n);
win2 = NewWindow("Transformed - White Transparent", img);