Legend size is calculated incorrectly with orientation 'v', yref 'container'

As mentioned in this PR comment (#7359), there appears to be a bug related to the legend size calculation (possible due to an incorrect margin calculation) when the orientation is set to v and the yref is set to container. You can see the issue by going through the following steps:

• Be on master
• Start up the developer dashboard
• Load the legend_horizontal_autowrap mock
• Note that it loads fine
• Edit the JSON for the mock to switch "orientation" to "v"
• Edit the JSON for the mock to add "yref" with a value of "container" under the "legend" key
• Reload the mock
• Note that the legend gets scrunched up into the top right corner of the plot
• Switch to this branch
• Reload the mock
• Note that an error is produced (due to a bad reaction to a negative margin value)

An example of the bug is seen in the following image:

The legend height should be calculated using the plot height (the "container"), but something is going wrong in this configuration.

developer dashboard を起動して legend_horizontal_autowrap mock を読み込み、legend orientation を "v"、yref を "container" に設定して問題を再現します。その再現を起点に、凡例のサイズとマージンの計算を追跡します。凡例が plot height をコンテナとして使用し、右上隅に押し込まれず、negative-margin error なしで再読み込みできれば完了です。