I have a chart that shows the number of items in a column chart per month. Now, I want to have a line in there that represents the relative percentage of the items in that month. E.g. I have 12 items in Jan, 16 in Feb, etc... Total items in that year: 160. So I want the line to show:
Jan: 12/160 = 7.5%
Feb: (12+16)/160 = 17.5%, etc...
How can this be achieved?