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.

启动 developer dashboard 并加载 legend_horizontal_autowrap mock，然后将 legend orientation 设置为 "v"、将 yref 设置为 "container" 来复现该问题。根据这次复现跟踪图例大小和边距的计算过程。完成标准是图例使用 plot height 作为其容器，不会被挤压到右上角，并且重新加载时不会出现 negative-margin error。