Making a drawing in the background of a SmileLab plot, like for any graphic view, is as straightforward as drawing in a graphic window. The only difference is that you pass to BeginFigure the reference to the graphic view.

If at first your drawing is not displayed, maybe you have an opaque white background (the fill color property of the graphic view): the background color is applied over the background graphic.

Import script

-- create random data
set xs to createarray 100
set ys to randomarray 100 range {-1, 1}
set ys to runningsum ys
-- plot data
set c to QuickCurve(xs, ys, "Random walk")
set c's pen color to 4
set c's pen width to 2
-- get reference to plot view
set the_plot_view to c's container
-- by default its background color is opaque white:
set the_plot_view's fill color to {0, 0, 0, 0}
-- prepare data for drawing
-- compute middle of plot
set {i0, j0, di, dj} to plot frame of the_plot_view
set ct to {i0 + di / 2, j0 + dj / 2}
-- launch drawing
BeginFigure(the_plot_view)
TextMoveTo(ct)
SetFillColor({0, 0, 0, 0.15})
SetTextSize(di / 5)
SetTextFont("Times New Roman")
DrawString("[h]Classified")
EndFigure()