Using files for graphics

import sys

'''
    sample data file contains the four lines:

         60  20
         20  60
        -60  20
         20 -60
'''

# First step:  get a file
filename = (sys.argv+[''])[1]
if not filename:
    raise ValueError('Use: '+sys.argv[0]+' data_file_name')

# Second step: read the file
with open(filename,'r') as inf:
    lines = inf.readlines()

# Third step: convert the data to numbers.
points =[list(map(float,line.split())) for line in lines]

# step 4 is optional, step 5 required:
# these two steps are intertwined.
def adjust(data,desired_minimum,desired_maximum):
    # improvement.  implement a equation of line to reduce passes through data
    n = min(data)
    data = [datum-n for datum in data]
    x = max(data)
    span = desired_maximum - desired_minimum
    m = span/x
    return [desired_minimum+datum*m for datum in data]

(x,y,) = zip(*points)                   # required, step 5

x = adjust(x,50,250)                    # optional scale and translate x
y = adjust(y,50,250)                    # optional scale and translate y

# finish step 5.  create_polygon requires a flat list.
points = []
for xy in zip(x,y):
    points.extend(xy)

# Sixth step: Display and enjoy
import Tkinter # import Tkinter is slow.  That's why I put it after possible error conditions.
canvas=Tkinter.Canvas()
canvas.create_polygon(*points)
canvas.pack()
canvas.mainloop()