使用pyqtgraph.PlotCurveItem绘制波形图时怎么正填充？

使用pyqtgraph.PlotCurveItem绘制波形图时，设置fillLevel 参数后，正负都填充，我现在只想y>0时进行填充，y<0时绘制波形，应该怎么办？代码如下：

from PySide6.QtGui import *
from PySide6.QtCore import *
from PySide6.QtWidgets import *
import numpy as np
import sys
import pyqtgraph as pg
class plotWindow(QMainWindow):
    def __init__(self, inputPath = None, parent = None):
        super().__init__(parent)
        self.resize(1200,800)
        pg.setConfigOption('background', 'w')
        pg.setConfigOption('foreground', 'k')
        pg.setConfigOption('useNumba', True)
        pg.setConfigOption('leftButtonPan', True) # 左键拖动
        pg.setConfigOption('antialias', True) # 抗锯齿
        self.plot = pg.PlotWidget()
        self.setCentralWidget(self.plot)
        self.plot.disableAutoRange(True)
        x = np.arange(6000)
        curves = []
        for i in range(500):
            y = np.random.uniform(-1,1,6000)

            curve_fill = pg.PlotCurveItem(x, y,
                        pen = ({'color': 'k', 'width': 0.2}), 
                        fillLevel = 0, 
                        brush= (0,0,0,255),
                        skipFiniteCheck=True)
            curve_fill.setPos(0, i)
            curves.append(curve_fill)
        for c in curves: 
            self.plot.addItem(c) 
        self.plot.setYRange(0, 500, padding = 0)
        self.plot.setXRange(0, 6000, padding = 0)  
if __name__ == '__main__':
    app = QApplication(sys.argv)
    window = plotWindow()
    window.show()
    sys.exit(app.exec())