import numpy as np
import holoviews as hv
from holoviews import opts

hv.extension('matplotlib')

xs = np.linspace(-5,5,100)
ys = -(xs-2)**2
vline = hv.VLine(xs[ys.argmax()])

(hv.Curve((xs,ys)) * vline).opts(
    opts.Curve(color='#D3D3D3'),
    opts.VLine(color='red', linewidth=6))