Merge ff4535c56bb91a61a9fc985c0038251485f35003 into 6e0ea441e40395ce5e8fe5a1b22d7829a0adb583

src/dataframe/df.rs

@@ -1,5 +1,7 @@
 use crate::frame::{Frame, RowIndex};
+use crate::matrix::Matrix;
 use std::collections::HashMap;
+use std::fmt;
 
 /// Represents a column in a DataFrame, holding data of a specific type.
 /// Each variant wraps a `Frame<T>` where T is the data type.
@@ -284,8 +286,6 @@ impl DataFrame {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::frame::Frame;
-    use crate::matrix::Matrix;
     use chrono::NaiveDate;
 
     // Helper for dates