Facets: "free x-scale" has no effect on histogram.

[alshan]

See discussion: https://github.com/JetBrains/lets-plot/issues/955#issuecomment-2191425769

Two ways this issue can be fixed:

• Introduce an option similar to "trim" in geom_density() (in this case historgram can't be stacked)
• Remove empty bins from the data (by default or optionally).