docs: def json_to_url(json, symbol):
      """ Converts a JSON to a URL by the Poloniex API 
  
      Args:
          json: JSON data as a list of dict dates, where the keys are
              the raw market statistics.
          symbol: String of currency pair, like a ticker symbol.
  
      Returns:
          String URL to Poloniex API representing the given JSON.
      """
      start = json[0]['date']
      end = json[-1]['date']
      diff = end - start
  
      # Get period by a ratio from calculated period to valid periods
      # Ratio closest to 1 is the period
      # Valid values: 300, 900, 1800, 7200, 14400, 86400
      periods = [300, 900, 1800, 7200, 14400, 86400]
  
      diffs = {}
      for p in periods:
          diffs[p] = abs(1 - (p / (diff / len(json)))) # Get ratio
  
      period = min(diffs, key=diffs.get) # Find closest period
      
      url = ('https://poloniex.com/public?command'
             '=returnChartData&currencyPair={0}&start={1}'
             '&end={2}&period={3}').format(symbol, start, end, period) 
      return url