Adding percentage as a second y-axis on chart?

I don't know that it is embarrasingly simple (in some ways, it is suprisingly difficult). I would not be surprised if there are other charting applications that would do this easily, but, in Excel, it is not obvious how to do this.

I'm sure there are several approaches. I would probably do something like this proportional secondary axis technique: https://peltiertech.com/secondary-ax...tional-scales/ The tutorial is written for a line chart/scatter chart, but it should be fairly straitforward to combine the scatter chart for the "dummy" secondary axis with your column chart. Are you familiar with combining XY scatter charts and column charts? Can you piece this together, or do you need further help?

you don't absolutely need the XY chart labeler add-in. If you have access to 2013 or later, then this feature is built into Excel. You could also write your own VBA procedure that will add the desired data labels as text in cells. Rob Bovey's add-in is free and does this job very well for those of us late adopters who have not upgraded, yet.

I'm curious what you tried, and why it did not work. I would have expected something like:

1) Compute values for secondary axis in spreadsheet
1a) Enter a row of six cells, all containing the value 7.5 (0.5+number of categories in chart).
1b) Desired fractions/percentages for axis labels (I would expect 0, 0.1, 0.2, 0.3, ...) in six cells below 1a
1c) A formula to compute the absolute value associated with each axis label. Your percentages appear to be calculated as =C11/SUM($C$11:$I$11). This should be the same formula, rearranged to find C11. Assuming 1b was entered in C16:H16, this might be =SUM($C$11:$I$11)*C16
2) Add "dummy axis" to chart
2a) Select data -> add series -> values is C17:H17
2b) Select newly added data series -> Change chart type -> XY scatter markers and lines
2c) My version automatically added the secondary Y and X axes. If yours does not, add these axes.
2d) Format secondary X axis so the max is 7.5, which should move the "dummy axis" to the far right.
2e) Format secondary Y axis so that its min and max limits match the primary Y axis.
2f) Add data labels (using the chart labeler add-in, your own VBA procedure, or using the built in option in 2013 or later) to the dummy axis.
2g) Hide the secondary Y and X axes.

Hi

How did you want the percentage data displayed? Just as data labels, or as an actual plot on the chart? You could simply add the percentages as another series, plot it on a secondary axis and then set the series to no fill and add data labels. See attached example.