I don't think it will be necessary to create a calculated field to plot the join,
one can plot the plan directly, if the two datasets are blended or joined together.
In the workbook attached in the Forum Thread, an example of a blend and a join are given.
For the blend, I created a Custom Date of Month Value (using Custom Dates )
in each of the datasources, one on [Date] and one on [Month].
I used this to blend the two datasources together in Data>Edit Relationships,
along with [Day] and [Type] (please see screenshot below).
Then, you can just plot SUM(Plan) on the second axis.
This can also be accomplished with a join of the two datasources,
as shown in the second screen shot.
274032blend.twbx 48.7 KB