code: def test_overlay_ylabel(self):
      overlay = (Curve(range(10)) * Curve(range(10))).opts(
          ylabel="custom y-label"
      )
      axes = mpl_renderer.get_plot(overlay).handles["axis"]
      self.assertEqual(axes.get_ylabel(), "custom y-label")
  
imports: import numpy
  from holoviews.core import DynamicMap, HoloMap, NdOverlay, Overlay
  from holoviews.element import Curve, Scatter
  from ...utils import LoggingComparisonTestCase
  from .test_plot import TestMPLPlot, mpl_renderer
label_desc: Use image node_id 10 for calling the TestOverlayPlot obj's underlying member method code with example usage: obj.test_overlay_ylabel() without return types