If your objective is to see the Monthly Total in a visual, just drag the Date column to your visual and write a simple SUM measure to add the numbers in the Sales column. Why do you need a Monthly Total column in your base data table?
I didnt provide my end goal as I thought by getting the answer would point me in the right direction. So here is my end goal. I also have another table similar to the on below. It also only has the first two columns. My goal is to calculate the the difference of estimated and actual sales by month and then in the visualizer graph the difference by month.
Below are samples of the date and the desired outcome. Table 1 is the estimated sales and operations. It will have one line per month for each item. I need to some how indentify the sales row for each month.
The second table will have several lines per monthn and the number of lines per month can vary. I need to add each month up to get my actual sales
The last table is the desired out come. It has on line per month and it has subtracted the monthly estimated sales from the actual sales