If you have Excel 2010 and you are making a combo chart with two Bars and one line, 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, "All Targets" in column C and "% of Met" in column D together;
Step 2: Highlight the data in the data range A1:D6. 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 very 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 Line on the left navigation bar, then select "Line with Markers".
Step 6: Now the line is too low and only have one Y Axis on the left. Right click on the line chart, then select "Format Data Series" to make a secondary Y-Axis for the line chart (%);
Step 7: In the "Format Data Series" window, check "Secondary Axis" from the Series Options section;
Step 8: A combo chart with two Y-Axis is created. You can change the color, Axis value or any other elements to make it better.