docs: def alpha_from_cov(plotman, alpha_cov):
      '''Calculate alpha values from the coverage/2.5.
      '''
      abscov = np.abs(load_cov('inv/coverage.mag'))
      if alpha_cov:
          normcov = np.divide(abscov, 2.5)
          normcov[np.where(normcov > 1)] = 1
          mask = np.subtract(1, normcov)
          alpha = plotman.parman.add_data(mask)
      else:
          alpha = plotman.parman.add_data(np.ones(len(abscov)))
      return alpha, plotman