agg.path_storage.arc_to problem

Hi,
Exploring the agg.py module of the matplotlib library, I have encauntered that I cannot represent smooth arcs. For example, circle appears as octagon. Could you give me an idea how to manage this and what is the reason for such behaviour? Here is the code of the agg_test.py changed to draw arc, but not giving the desired result:

···

##########################
import matplotlib.agg as agg

width, height = 600,400
stride = width*4
buffer = agg.buffer(width, height, stride)

rbuf = agg.rendering_buffer()
rbuf.attachb(buffer)

red = agg.rgba8(255,0,0,255)
blue = agg.rgba8(0,0,255,255)
green = agg.rgba8(0,255,0,255)
black = agg.rgba8(0,0,0,255)
white = agg.rgba8(255,255,255,255)
yellow = agg.rgba8(192,192,255,255)

path = agg.path_storage()
path.move_to(10,10)
path.line_to(100,100)
path.line_to(200,200)
path.arc_to(100,100,0,0,1,100,200)
path.close_polygon()

stroke = agg.conv_stroke_path(path)
stroke.width(3.0)

pf = agg.pixel_format_rgba(rbuf)
rbase = agg.renderer_base_rgba(pf)
rbase.clear_rgba8(blue)

renderer = agg.renderer_scanline_aa_solid_rgba(rbase);
renderer.color_rgba8( red )

rasterizer = agg.rasterizer_scanline_aa()
rasterizer.add_path(stroke)

scanline = agg.scanline_p8()

agg.render_scanlines_rgba(rasterizer, scanline, renderer);

s = buffer.to_string()
print len(s)
import Image
im = Image.fromstring( "RGBA", (width, height), s)
im.show()

Thank you!
Regards,
Z.Danevska