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.

Démarre le developer dashboard et charge le legend_horizontal_autowrap mock, puis reproduis le problème en définissant legend orientation sur "v" et yref sur "container". Suis le calcul de la taille et de la marge de la légende à partir de cette reproduction. C’est terminé lorsque la légende utilise la plot height comme conteneur, n’est pas comprimée dans le coin supérieur droit et se recharge sans negative-margin error.