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.

Start the developer dashboard and load the legend_horizontal_autowrap mock, then reproduce the issue by setting legend orientation to "v" and yref to "container". Trace the legend size and margin calculation from that reproduction. Done means the legend uses the plot height as its container, is not scrunched into the top-right corner, and reloads without a negative-margin error.