class_docstr: Main spectrum plot
docstr: Update crosshair when mouse is moved
docstr_tok: ["Update","crosshair","when","mouse","is","moved"]
code: def mouse_moved(self, evt):
      """Update crosshair when mouse is moved"""
      pos = evt[0]
      if self.plot.sceneBoundingRect().contains(pos):
          mousePoint = self.plot.vb.mapSceneToView(pos)
          self.posLabel.setText(
              "<span style='font-size: 12pt'>f={:0.3f} MHz, P={:0.3f} dB</span>".format(
                  mousePoint.x() / 1e6, mousePoint.y()
              )
          )
          self.vLine.setPos(mousePoint.x())
          self.hLine.setPos(mousePoint.y())
  
imports: import collections, math
  from Qt import QtCore
  import pyqtgraph
label_desc: Use image node_id 21 for calling the SpectrumPlotWidget obj's underlying member method code with example usage: obj.mouse_moved(evt) without return types