See the Processing function triangle():
        https://processing.org/reference/triangle_.html

def triangle(self, x1, y1, x2, y2, x3, y3, color):
        """
        See the Processing function triangle():
        https://processing.org/reference/triangle_.html
        """
        self.context.set_source_rgb(*color)
        self.context.move_to(self.tx(x1), self.ty(y1))
        self.context.line_to(self.tx(x2), self.ty(y2))
        self.context.line_to(self.tx(x3), self.ty(y3))
        self.context.line_to(self.tx(x1), self.ty(y1))
        self.context.fill()