Use Month Type as a dimension in your chart, sum(Units) as your expression. No variables, no set analysis, all complexity in the script instead of the chart, which is where I prefer to keep it when practical.
This may not be the right solution for your specific requirement. Like the chart I suggest doesn't make it easy to then, say, add the difference in units between the months as another column. You might handle that by removing the Month Type dimension and having three expressions: