class_docstr: Main spectrum plot
code: def __init__(self, layout):
      if not isinstance(layout, pg.GraphicsLayoutWidget):
          raise ValueError(
              "layout must be instance of pyqtgraph.GraphicsLayoutWidget"
          )
  
      self.layout = layout
  
      self.main_curve = True
      self.main_color = pg.mkColor("y")
      self.persistence = False
      self.persistence_length = 5
      self.persistence_decay = "exponential"
      self.persistence_color = pg.mkColor("g")
      self.persistence_data = None
      self.persistence_curves = None
      self.peak_hold_max = False
      self.peak_hold_max_color = pg.mkColor("r")
      self.peak_hold_min = False
      self.peak_hold_min_color = pg.mkColor("b")
      self.average = False
      self.average_color = pg.mkColor("c")
      self.baseline = False
      self.baseline_color = pg.mkColor("m")
  
      self.create_plot()
  
imports: import collections, math
  from Qt import QtCore
  import pyqtgraph
label_desc: Use image node_id 1 to create a new SpectrumPlotWidget object  with example: obj = SpectrumPlotWidget(layout)