If you have two series of data and one of the series is on a different scale, it might be impossible to interpret the smaller series.
Data in series A on the excel chart above is really hard to compare and analyze. You can solve this by moving series B to a secondary axis.
Add a secondary axis
Right click on series B
Click "Format Data Series..."
Select "Secondary Axis"
Click Close button
If you are unlucky, one of the data series is hidden. In this case, series A is totally hidden behind series B, see chart above. You can change the transparency for series B but in my opinion it makes the chart better but not great.
Yes, you can see both data series but it is still a mess and the colors in the legend and the columns don't match, see chart above. Don't add transparency, try separating the two series instead. Instructions, below.
Add two blank series
This is the data I am working with.
Here is how to add a blank series.
Right click on chart
Click on "Select Data..."
Click "Add" button
Select a blank cell range, in this case D2:D7
Repeat step 3 - 6 to add a fourth blank series, you can use the same cell range. Then click OK button. The chart now looks like this: