Add option to format dates as UTC

Currently, Plotly offers the tickformat property to specify date formats. But the results are always shown in local time. I see no way to format the dates as UTC other than manually transforming all of the dates to offset based on the current time zone so they look like the equivalent UTC time. And obviously, this is a major pain.

D3 can certainly handle UTC dates, so Plotly really needs to expose something to allow dates to be shown as UTC.

まず、Plotly の tickformat における日付処理を追跡し、D3 の utcFormat の動作と比較します。UTC フォーマットのオプションをどこに配置すべきかを判断し、そのうえで、呼び出し側がデータを変換しなくても日付ラベルが UTC でレンダリングされることを検証します。