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 .

issue #2155 で参照されている plotly.js の動作を、plotly.py が受け付ける layout hovermode の値と比較し、現在 'compare' を拒否している検証から調査を始める。traces 間で期待される垂直方向の hover 動作を確認し、サポートされている値によって要求された表示が得られることを検証する。