It cannot be changed at this time. We'll implement a header font property for the next release.

Stoyan

At this time you can implement that by drawing page breaks from the DrawBackground event handler:

public void drawBackground(DiagramEvent e)
{
	Rectangle2D r = diagram.getBounds();
	double unitScale = 25.4 / 72;	// if MeasureUnit is millimeters

	double w = defaultFormat.getImageableWidth() * unitScale;
	double h = defaultFormat.getImageableHeight() * unitScale - 15 * unitScale /* header area */;

	Graphics2D g = e.getGraphics();
	g.setPaint(new Color(128, 128, 128, 100));

	// draw vertical page delimiters
	for (double x = r.getMinX(); x < r.getMaxX(); x += w)
		g.draw(new Line2D.Double(x, r.getMinY(), x, r.getMaxY()));


	// draw horizontal page delimiters
	for (double y = r.getMinY(); y < r.getMaxY(); y += h)
		g.draw(new Line2D.Double(r.getMinX(), y, r.getMaxX(), y));
}; 

You might want to check some boolean flag and not draw the lines when actually printing. Also if your diagrams are large, you could limit line drawing to the e.getVisibleRect() rectangle to avoid unnecessary drawing in the off-screen part of the diagram.

The current 15-points height subtracted for the header area will depend on the header font when we add property for it.

I hope that helps,
Stoyan