Left edge of the plot area.
Right edge of the plot area.
Height of the top margin (the plot area starts here).
Measured width of the label text.
Horizontal space between the text and the box edges.
Preferred box height.
The box, with width and height of at least 0.
Places the trigger level label in the top margin, above the plot area, so it never covers the trace.
The box starts at the left edge of the plot area and is centred vertically in the top margin. It is never wider than the plot area and never taller than the margin; text that does not fit is clipped by the caller.