If you have Excel 2010 and you are making a combo chart with 2 Y-Axis, please follow the steps below. Excel 2016 has made this much easier, if you are working with Excel 2016, please refer here for details.
Step 1: Suppose we are making a chart with the data in the Table below, and we want to combine the number of "Met" in column B and "% of Met" in column C together;
Step 2: Highlight the data in the data range A1:C6. In the "Insert" Tab from the Ribbon, click "Column", then select "Clustered Column".
Step 3: A combo chart will be created. The percentage columns are short and we need to change them to a line chart.
Step 4: To change Columns to Line chart: Right click on the short columns, and select "Change Series Chart Type" from the dialog box;
Step 5: In the "Change Chart Type" window, select Column on the left navigation bar, then select "Line with Markers".
Step 6: Now the combo chart should like the one below, the line chart is low and only have one Y Axis on the left.
Step 7: To make a secondary Y-Axis for the line chart (%): right click on the line chart, and select "Format Data Series";
Step 8: In the "Format Data Series" window, check "Secondary Axis" from the Series Options section;
Step 9: A combo chart with two Y-Axis is created. You can change the color, Axis value or any other elements to make it better.