This is different than the Processing function arc():
        https://processing.org/reference/arc_.html

        Use the Cairo arc() function to draw an arc with a given line thickness.

def arc(self, x, y, width, height, start, end, color, thick=1, _=None):
        """
        This is different than the Processing function arc():
        https://processing.org/reference/arc_.html

        Use the Cairo arc() function to draw an arc with a given line thickness.
        """
        thick *= 4
        self.context.set_source_rgb(*color)
        self.context.save()
        self.context.translate(self.tx(x+(width/2)), self.ty(y+(height/2)))
        self.context.scale(self.tx(width/2), self.ty(height/2))
        self.context.arc(0.0, 0.0, 1.0 - (self.tx(thick)/2),
            (2*math.pi*start)/360,
            (2*math.pi*end)/360
        )
        self.context.set_line_width(self.tx(thick))
        self.context.stroke()
        self.context.restore()