For multi-series charts we perform an equi-join between the series on the X axis, therefore if the X axis values do not match each other then they are omitted. This was done for query performance reasons since a full outer join is required otherwise. You need to make sure that you return matching X axis data for both series.
We may consider adding an option in the future for you to select this full outer join option in the chart in a future release. It would help if you could argue your case as to why you need this that would be helpful.