old_contents: import numpy as np
  from matplotlib import pyplot as plt
  import heat1d
  
  runner = heat1d.Heat1D_SDC()
  
  def case(dt):
      runner.dt = dt
      t, r = runner.convergence(runner.run())
      plt.plot(t,r, ".-")
  
  dts = [ 0.01, 0.05, 0.1 ]
  for dt in dts:
      case(dt)
  
  plt.xlabel("$t$")
  plt.ylabel("absolute residuals")
  plt.savefig("plot/heat1d/sdc_dt")
  
new_contents: import numpy as np
  from matplotlib import pyplot as plt
  import heat1d
  
  runner = heat1d.Heat1D_SDC()
  
  def case(dt):
      runner.dt = dt
      t, r = runner.convergence(runner.run())
      plt.plot(t,r, ".-")
  
  dts = [ 0.01, 0.05, 0.1 ]
  for dt in dts:
      case(dt)
  
  plt.xlabel("$t$")
  plt.ylabel("absolute residuals")
  plt.savefig("plot/heat1d/sdc_dt.pdf")
  
subject: fix sdc_dt plot