[Lens] Thresholds: first automatic static value should be the 75% of the overall data bounds

[dej611]

Describe the feature:

Compute the default static value from all the dimensions for the first available axis rather than just one dimension, in order to show it at 75% of the current chart workspace.

[elasticmachine]

Pinging @elastic/kibana-vis-editors (Team:VisEditors)

[dej611]

@MichaelMarcialis I've done a first attempt at this, but there is probably some more in depth investigation to do.

The overall data bounds is known only by the visualization at the very end of it: this is computed based on a composition of chart attributes.
Because in Lens the XY visualization can be composed by multiple layers and chart types, the user may end up in a scenario where there are mixed stacked, unstacked and percentage charts in the same visualization.