Function correlation_matrix 

pub fn correlation_matrix(
    columns: &[Vec<f64>],
    method: CorrelationMethod,
) -> Result<Vec<Vec<f64>>, StatsError>

Correlation matrix of columns (each inner slice is one variable’s values, all the same length). The diagonal is exactly 1.0; any pair with a constant column yields f64::NAN in that cell (so a chart can render it as a distinct “undefined” color) rather than erroring out the whole matrix.

Errors

• StatsError::EmptyInput if there are no columns.
• StatsError::LengthMismatch if the columns are not all the same length.