[Lens] Allow partial config to be kept when switching from metric/table to Cartesian charts

[AlonaNadler]

**Kibana version:**7.5.x
When fields are placed in configurator and users change charts the configurator clears the selected fields.

When does it happen:

• When moving from metric visualization to any of the xy charts
• When moving from the table to any of the xy charts
  

Lens should persist the fields in the configurator so when moving between the chart or going back to the chart it will stay populated

[wylieconlon]

This happens mostly because we don't treat it as a valid bar chart without an X axis. I'm not sure what you were expecting, but these are some of the options we could consider:

a) The metric is assigned to the Y axis, and there is an error saying an X axis is required
b) A suggestion would be selected, such as "metric over time"

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[AlonaNadler]

Showing an error is not great.

I prefer
b) A suggestion would be selected, such as "metric over time"

[rayafratkina]

Option A would be part of a more general effort to add better error states. We can also make a suggestion to the user about how to address this.

Possible complication: state management issues may complicate this.

[flash1293]

This is fixed in the latest version of Lens - it shows an xy chart without an x axis