# mwaskom__seaborn-3407
# Loki Mode Multi-Agent Patch
# Attempts: 1

Based on the issue description, traceback, and architect's analysis, I can generate a patch for seaborn's axisgrid.py. The problem occurs because when a DataFrame has MultiIndex columns, the column names are tuples like `('A', '1')`, but when pandas tries to access `self.data[var]` where `var` is such a tuple, it's incorrectly interpreted.

The fix should flatten MultiIndex columns to string representations at the start of `PairGrid.__init__`.

```
--- a/seaborn/axisgrid.py
+++ b/seaborn/axisgrid.py
@@ -1220,6 +1220,12 @@ class PairGrid(Grid):
         """
         super().__init__()
 
+        # Handle MultiIndex columns by flattening them to strings
+        if isinstance(data.columns, pd.MultiIndex):
+            data = data.copy()
+            data.columns = [
+                "_".join(str(c) for c in col) for col in data.columns
+            ]
+
         # Sort out the variables that define the grid
         numeric_cols = self._find_numeric_cols(data)
         if hue in numeric_cols:
```

This patch:
1. Checks if the DataFrame has MultiIndex columns using `isinstance(data.columns, pd.MultiIndex)`
2. Creates a copy of the data to avoid modifying the original DataFrame
3. Flattens the MultiIndex columns by joining all levels with underscores, converting each level to string to handle non-string level values