class_docstr: Main spectrum plot
docstr: Create spectrum persistence curves
docstr_tok: ["Create","spectrum","persistence","curves"]
code: def create_persistence_curves(self):
      """Create spectrum persistence curves"""
      z_index_base = 600
      decay = self.get_decay()
      self.persistence_curves = []
      for i in range(self.persistence_length):
          alpha = 255 * decay(i + 1, self.persistence_length + 1)
          color = self.persistence_color
          curve = self.plot.plot(
              pen=(color.red(), color.green(), color.blue(), alpha)
          )
          curve.setZValue(z_index_base - i)
          self.persistence_curves.append(curve)
  
imports: import collections, math
  from Qt import QtCore
  import pyqtgraph
label_desc: Use image node_id 8 for calling the SpectrumPlotWidget obj's underlying member method code with example usage: obj.create_persistence_curves() without return types