title: Google Sheets: Chart: Increase Moving Average Trendline Period
body: <p>I've set a Trendline for a Chart Series to Moving Average, but the Period only accepts a maximum of 10 periods.</p>
  <p>The data is provided as Weekly Stock Prices, using GOOGLEFINANCE(), for the last 10 years.</p>
  <p>I would like to see a trendline over a longer period, such as 26 (weeks). Ideally, I would specify this Period in a sheet cell, so I can change it depending on the stock I'm interested in.</p>
  <p>One option I thought might work is to add an extra column that works out an average, and use that as another Series? However I don't know how to specify the number of Periods in such a calculated Average().</p>
  <p>Is there any method to do this?</p>