[Lens] Move default legend position to bottom for charts

[dej611]

Describe the feature:

The current legend position in charts in the right hand side one:

while this position is functional, it "removes" a lot of charting space by default to the workspace, in particular for date histogram scenarios where more width is quite important.

While there's the option to manually move the legend to another position, I would propose to change the default behaviour here to bottom for XY and Heatmap charts:

Maybe to consider also the Mosaic case, which is a peculiar partition chart.
I do not believe other chart type needs such big change.

Some extra thoughts:

• provide a migration for existing chart to explicit right hand side position or let them move to bottom? (keep default config)
• solve also [Heatmap] Legend is taking up too much space when positioned above/bottom the chart #152076 for heatmap case

[elasticmachine]

Pinging @elastic/kibana-visualizations (Team:Visualizations)

[drewdaemon]

This seems to me like a great improvement. Many users are defaults-users. This has the potential to improve out-of-the-box aesthetics.

I guess the only downside is that it can lead to misaligned charts if the legends take up different amounts of vertical space.

Not sure how big a drawback this would be

[markov00]

In order to provide better transparency of priorities, issues that will not be prioritized within the next 24 months are being closed.

Tracking request in Lens general improvements ice box #184459