def archive_e(charger_id, transaction_id, meter_values):
"""
Store MeterValues for a charger/transaction as a dated file for graphing.
"""
date_str = datetime.utcnow().strftime("%Y-%m-%d")
base = gw.resource("work", "etron", "graphs", charger_id)
os.makedirs(base, exist_ok=True)
# File name: <date>_<txn_id>.json (add .json for safety)
file_path = os.path.join(base, f"{date_str}_{transaction_id}.json")
with open(file_path, "w") as f:
json.dump(meter_values, f, indent=2)
return file_path