"Shared Axis" for two date columns on Clustered Line Graph?

Hello,

I am hoping someone can suggest a measure to use as a shared axis in a Clustered Line and Bar Graph visual.

I have a table with two date columns, and I would like to display the count of the dates in each column in a bar graph, grouped by month across a multi-year scale. So for example, if there are 4 dates which occur in April '15 in column 1, and 2 dates in April '15 in column 2, then the bars in the graph above April would be valued at 4 and 2, with the X-axis expanding across the full timeline covered in the columns.

Here's a mockup visual of what I am looking for:

I've tried using a few different methods, such as duplicating one of the columns and transforming it into a Start of Month column and using that as the axis, but the numbers are always wrong in at least one of the counts illustrated by the bars.

Any help would be very much appreciated and rewarded with kudos!