ZeroDivisionError: float division

Iv got an error , when all the value in the list is the same,
" b=points[a+1]-points[a]"=0
then "list_progre"=[0,0,0,0,0,0,0]
and then : "progres=plot(dt_date, list_progre, '')" made me an error : "var = dv/max(abs(vmin), abs(vmax)) ZeroDivisionError: float division"

what can i do to stop this error

thx

The program test:

from pylab import *

list_progre=[]
dt_date=[12,13,14,15,16,17,18,19]
points=[25,25,25,25,25,25,25,25]
num_fig=1
nom='test'
chemin_fichier='C:\Python24\uni_33\\'

long_list=len(points)-1
a=0
while a<long_list :
    b=points[a+1]-points[a]
    list_progre.append(b)
    a=a+1

f=figure(num_fig,figsize=(8,6), dpi=72)
subplot(211)
classt=plot(dt_date, points, '')
setp(classt, color='r', linewidth=2.0)
title('Nombre de point au classement general')
xlabel('time')
ylabel('points')
del(dt_date[0])
subplot(212)
progres=plot(dt_date, list_progre, '')
setp(progres, color='g', linewidth=1.0)
title('Progression')
nom_fichier=string.replace(nom,' ','_')
rep_fichier=chemin_fichier+'stat_img\\'
savefig(rep_fichier+nom_fichier, dpi=50)
show()
#f.clf()