[Lens] Allow customizing the whitespace in Lens visualizations

[davismcphee]

Describe the feature:

It would be useful to allow customizing the whitespace in Lens visualizations when consuming them as embeddables. In Discover we are using the embeddable with noPadding, which eliminates CSS padding, but there is additional whitespace in the canvas that affects the alignment, and currently we don't seem to have a way to control it:

Some context provided by a member of the Elastic Charts team:

...that white space is either determined by the theme.chartPaddings or theme.chartMargins, talks to reduce this start here. Or if those are theme properties are both zero and you still see space could be the title is set to '' still taking up space, you have axes paddings (see example), or the tick positioning logic is pushing the axis out to fit the text on the x-axis.

Describe a specific use case for the feature:

Supporting this would allow us to better align the Lens histogram used in Discover.

[elasticmachine]

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

[markov00]

I believe this is already possible as you can customize/override embeddable chart Settings.
Please reopen if required.