def calculate_top_margin(self):
		"""
		Calculate the margin in pixels above the plot area, setting
		border_top.
		"""
		self.border_top = 5
		if self.show_graph_title:
			self.border_top += self.title_font_size
		self.border_top += 5
		if self.show_graph_subtitle:
			self.border_top += self.subtitle_font_size