Retrieves historical price data (OHLCV) for a given symbol.
Args:
symbol: The ticker symbol (e.g., 'AAPL', 'BTC-USD').
interval: Data interval. Valid values: "1m", "2m", "5m", "15m", "30m", "60m", "90m", "1h", "1d", "5d", "1wk", "1mo", "3mo".
period: Data period to download. Valid values: "1d", "5d", "1mo", "3mo", "6mo", "1y", "2y", "5y", "10y", "ytd", "max".
visualize: If True, returns a candlestick chart instead of raw data.
Returns:
JSON string of OHLCV data or base64-encoded candlestick chart.