I a drawing a graph using Cairo (pycairo specifically) and I need to know how can I draw text inside a circle without overlapping it, by keeping it inside the bounds of the circle. I have this simple code snippet that draws a letter "a" inside the circle:

'''
Created on May 8, 2010

@author: mrios
'''
import cairo, math

WIDTH, HEIGHT = 1000, 1000

#surface = cairo.PDFSurface ("/Users/mrios/Desktop/exampleplaces.pdf", WIDTH, HEIGHT)
surface = cairo.ImageSurface (cairo.FORMAT_ARGB32, WIDTH, HEIGHT)
ctx = cairo.Context (surface)

ctx.scale (WIDTH/1.0, HEIGHT/1.0) # Normalizing the canvas


ctx.rectangle(0, 0, 1, 1) # Rectangle(x0, y0, x1, y1)
ctx.set_source_rgb(255,255,255)
ctx.fill()

ctx.arc(0.5, 0.5, .4, 0, 2*math.pi)
ctx.set_source_rgb(0,0,0)
ctx.set_line_width(0.03)
ctx.stroke() 

ctx.arc(0.5, 0.5, .4, 0, 2*math.pi)
ctx.set_source_rgb(0,0,0)
ctx.set_line_width(0.01)
ctx.set_source_rgb(255,0,255) 
ctx.fill()
ctx.set_source_rgb(0,0,0)

ctx.select_font_face("Georgia",
            cairo.FONT_SLANT_NORMAL, cairo.FONT_WEIGHT_BOLD)
ctx.set_font_size(1.0)
x_bearing, y_bearing, width, height = ctx.text_extents("a")[:4]
print ctx.text_extents("a")[:4]
ctx.move_to(0.5 - width / 2 - x_bearing, 0.5 - height / 2 - y_bearing)
ctx.show_text("a")

surface.write_to_png ("/Users/mrios/Desktop/node.png") # Output to PNG

The problem is that my labels have variable amount of characters (with a limit of 20) and I need to set the size of the font dynamically. It must fit inside the circle, no matter the size of the circle nor the size of the label. Also, every label has one line of text, no spaces, no line breaks.

Any suggestion?