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.

Starte das developer dashboard und lade den legend_horizontal_autowrap mock, reproduziere dann das Problem, indem du die legend orientation auf "v" und yref auf "container" setzt. Verfolge anhand dieser Reproduktion die Berechnung von Legendengröße und Rand. Als erledigt gilt die Aufgabe, wenn die Legende die plot height als ihren Container verwendet, nicht in die obere rechte Ecke gequetscht wird und ohne einen negative-margin error neu geladen wird.