text: from vedo import *
  from vedo.pyplot import histogram, plot
  
  cmap = 'nipy_spectral'
  alpha = np.array([0, 0, 0.05, 0.2, 0.8, 1])
  
  vol = Volume(dataurl+"embryo.slc")
  vol.cmap(cmap).alpha(alpha).addScalarBar3D(c='white')
  xvals = np.linspace(*vol.scalarRange(), len(alpha))
  
  fig = histogram(vol, logscale=True, c=cmap, ac='white')
  fig+= plot(xvals, alpha*max(fig.frequencies), '--ow', like=fig).z(1)
  
  show([
        (vol, Axes(vol, c='w'), f"Original Volume\ncolor map: {cmap}"),
        (fig, "Voxel scalar histogram\nand opacity transfer function")
       ],
       N=2, sharecam=False, bg=(82,87,110), zoom=1.1,
  ).close()
  