In a previous post here, I listed how you can create calculations to cover current month and previous month values. I also showed you how to add the previous month as a reference line on your bar chart.
You may have noticed that if you have more than one column, the reference lines almost overlap each other as shown below;
In order to fix this, remove the reference line from your bar chart.
Add your Previous Month calculated field to your Rows shelf.
Make it a Dual Axis with the Current Month calculated field.
Make the Current Month Mark Type a Bar.
Change the Previous Month Mark Type to a Gantt Bar!
Your Previous Month calculated field will display as a reference lines on top of your bars! They will not be as wide. You can also adjust the size of the bars to be smaller using the size slider. You can change the opacity of these bars if needed.