Function prepare_chart_x_range 

pub fn prepare_chart_x_range(
    lf: &LazyFrame,
    schema: &Schema,
    x_column: &str,
    row_limit: usize,
) -> Result<ChartXRangeResult>

Loads only the x column and returns its min/max (for axis display when no y is selected). Limits to row_limit rows then scans for min/max (single column materialized).