Aggregating month data from two SharePoint lists

Hi, new to Power BI here - apologies for my lack of knowledge. I am generating a report using two separate SharePoint 2013 lists, which deal with three different categories of documents (1 on the first list and 2 on the second). Each document type has a separate "Month" column in its list, denoting the month of the year that an individual document was sent. I have also created a measure which calculates the percentage of documents that were sent on time (number of documents sent on time/total number of documents sent).

When I plot this sent on time measure for one document type against the appropriate "Month" metric (and sort the Month by a MonthNumber column to get them to appear in the correct order), I get the following bar chart:

This works fine, but if I try to introduce another document type's sent on time measure to the same bar chart, I get the following:

For the second document type (in blue), only the overall percentage for all time is shown in each month. This is despite bringing in the other "Month" metric for B-type documents in as an axis. I presume this is because the "Month" data comes from two separate columns on two separate lists and so cannot be treated "as one"and/or you can only have one X axis on a bar chart. Is there a way to aggregate (not sure if that's the correct term) the Month data so I can achieve a bar chart which something that looks like this (created in Excel):