provide hovermode compare

I want hover a position show all data accross vertical line, like :

I found javascript version have this https://github.com/plotly/plotly.js/issues/2155#issuecomment-351393616

But hovermode='compare' got error

ValueError: 
    Invalid value of type 'builtins.str' received for the 'hovermode' property of layout
        Received value: 'compare'

    The 'hovermode' property is an enumeration that may be specified as:
      - One of the following enumeration values:
            ['x', 'y', 'closest', False]

PS: I really think bokeh's hovertool is much better than plotly , at least they put all hover relative things in one place with examples , easy to found what you want, simply add mode='vline' would work

HoverTool(
    tooltips=[
        ( 'date',   '@date{%F}'            ),
        ( 'close',  '$@{adj close}{%0.2f}' ), # use @{ } for field names with spaces
        ( 'volume', '@volume{0.00 a}'      ),
    ],
    formatters={
        'date'      : 'datetime', # use 'datetime' formatter for 'date' field
        'adj close' : 'printf',   # use 'printf' formatter for 'adj close' field
                                  # use default 'numeral' formatter for other fields
    },
    # display a tooltip whenever the cursor is vertically in line with a glyph
    mode='vline
')

plotly document contain many things but didn't group well, find a solution is not easy .

比较 issue #2155 中引用的 plotly.js 行为与 plotly.py 接受的 layout hovermode 值，从当前拒绝 'compare' 的验证开始。确认 traces 之间所需的垂直 hover 行为，并验证受支持的值会产生所请求的显示。