我尝试了所有三个后端，但没有显示任何图表。一个例子是：
!pip install -q holoviews
import holoviews as hv
from holoviews import opts
hv.extension('matplotlib')
# build a dataset where multiple columns measure the same thing
stamp = [.33, .33, .34, .37, .37, .37, .37, .39, .41, .42,
.44, .44, .44, .45, .46, .49, .49]
postcard = [.20, .20, .21, .23, .23, .23, .23, .24, .26, .27,
.28, .28, .29, .32, .33, .34, .35]
group = "U.S. Postage Rates (1999-2015)"
stamp = hv.Curve(stamp, vdims='Rate per ounce', label='stamp', group=group)
postcard = hv.Curve(postcard, …