Enhance incremental data refresh

tests/test_refresh_data.py

@@ -42,3 +42,33 @@ def test_refresh_data_writes_series(tmp_path, monkeypatch):
             for row in conn.execute("SELECT name FROM sqlite_master WHERE type='table'")
         }
     assert {"A", "B"} <= tables
+
+
+def test_refresh_data_appends_only_new_rows(tmp_path, monkeypatch):
+    calls = []
+
+    def fake_reader(name, src, start, end):
+        calls.append((name, start, end))
+        dates = pd.date_range(start, end)
+        return pd.DataFrame({name: range(len(dates))}, index=dates)
+
+    monkeypatch.setattr(refresh_data, "DataReader", fake_reader)
+    monkeypatch.setattr(refresh_data, "DATA_PATH", tmp_path)
+    db_file = tmp_path / "fred.db"
+    monkeypatch.setattr(refresh_data, "DB_FILE", db_file)
+
+    # Pre-populate DB with two days of data
+    pre_df = pd.DataFrame({"A": [1, 2]}, index=pd.date_range("2000-01-01", periods=2))
+    with sqlite3.connect(db_file) as conn:
+        pre_df.to_sql("A", conn, if_exists="replace", index_label="date")
+
+    refresh_data.main(
+        ["--series", "A", "--start", "2000-01-01", "--end", "2000-01-04"]
+    )
+
+    # Should only fetch after 2000-01-02
+    assert calls == [("A", "2000-01-03", "2000-01-04")]
+
+    with sqlite3.connect(db_file) as conn:
+        df = pd.read_sql("SELECT * FROM A", conn, parse_dates=["date"], index_col="date")
+    assert len(df) == 4