This dialog is available from the Graphs / Options / Moving Average Weeks... menu command under the Graphs window.
This dialog box allows you to specify how many moving averages to display for the moving average graphs and the length of time for each average. Zero to six averages can be displayed, each for 1 to 9,999 weeks. For each average to display check the "Enable" button and specify the number of weeks for that average. For large values of weeks to average, the time to refresh the screen will increase, due to the longer calculation times.
Each "week" is 5 days of prices, or periods. So, for a 50 day moving average you would choose 10 weeks. A 200 day moving average is 40 weeks.