I tried to create a measure which Provide us the values you want. but then I noticed that since summation of all Group percentages in each month comes out to be 100%, therefore the Stacked bar will always shows what it is showing now.
For example. ( below screesnhot shows the % of all Group on Monday July,01
Therefore your stacked bar also shows the % of grand total, where grand total is always 100 %
The only correct way to show this on the graph is the top right Graph in your report.
Or you can show the Top left bar graph with DAX Measure in a Tooltip so when a user hover over it, they will able to see the % count for each of the group in each month.