docs: def make_multi_qq_plots(arrays, key_text):
      """Make a quantile-quantile plot comparing multiple sets of events and models.
  
      *arrays*
        X.
      *key_text*
        Text describing the quantile-quantile comparison quantity; will be
        shown on the plot legend.
      Returns:
        An :class:`omega.RectPlot` instance.
  
      *TODO*: nothing about this is Sherpa-specific. Same goes for some of the
      plotting routines in :mod:`pkwit.environments.casa.data`; might be
      reasonable to add a submodule for generic X-ray-y plotting routines.
  
      *TODO*: Some gross code duplication here.
  
      """
      import omega as om
  
      p = om.RectPlot()
      p.addXY([0, 1.], [0, 1.], '1:1')
  
      for index, array in enumerate(arrays):
          kev, obs, mdl = array
          c_obs = np.cumsum(obs)
          c_mdl = np.cumsum(mdl)
  
          mx = 0.5 * (c_obs[-1] + c_mdl[-1])
          c_obs /= mx
          c_mdl /= mx
  
          p.addXY(c_mdl, c_obs, '%s #%d' % (key_text, index))
  
      # HACK: this range of numbers is chosen to give reasonable sampling for my
      # sources, which are typically quite soft.
      #
      # Note: this reuses the variables from the last loop iteration.
      locs = np.array([0, 0.05, 0.08, 0.11, 0.17, 0.3, 0.4, 0.7, 1]) * (kev.size - 2)
      c0 = 1.05
      c1 = 1.1
  
      for loc in locs:
          i0 = int(np.floor(loc))
          frac = loc - i0
          kevval = (1 - frac) * kev[i0] + frac * kev[i0+1]
          mdlval = (1 - frac) * c_mdl[i0] + frac * c_mdl[i0+1]
          obsval = (1 - frac) * c_obs[i0] + frac * c_obs[i0+1]
          p.addXY([mdlval, mdlval], [c0, c1], '%.2f keV' % kevval, dsn=2)
          p.addXY([c0, c1], [obsval, obsval], None, dsn=2)
  
      p.setLabels('Cumulative rescaled model', 'Cumulative rescaled data')
      p.defaultKeyOverlay.vAlign = 0.3
      return p