code: def create_lifetime_chart(self, classname, filename=''):
          
          try:
              from pylab import figure, title, xlabel, ylabel, plot, savefig
          except ImportError:
              return HtmlStats.nopylab_msg % (classname+" lifetime")
  
          cnt = []
          for tobj in self.index[classname]:
              cnt.append([tobj.birth, 1])
              if tobj.death:
                  cnt.append([tobj.death, -1])
          cnt.sort()
          for i in range(1, len(cnt)):
              cnt[i][1] += cnt[i-1][1]
              #if cnt[i][0] == cnt[i-1][0]:
              #    del cnt[i-1]
  
          x = [t for [t,c] in cnt]
          y = [c for [t,c] in cnt]
  
          figure()
          xlabel("Execution time [s]")
          ylabel("Instance #")
          title("%s instances" % classname)
          plot(x, y, 'o')
          savefig(filename)
  
          return self.chart_tag % (os.path.basename(filename))
docs: Create chart that depicts the lifetime of the instance registered with
          `classname`. The output is written to `filename`.