How to create a Line Chart Trendline with multiple data series

I understand that scatter plots allow trendlines over multiple data series within the same chart.

However, scatter plots do not allow x-axis labels to be populated with text.

So, I have used a line chart (which allows text x-axis labels, e.g. January, February, etc), that has two data series.

How can I create a trendline in a line chart that combines the data from both series, the same way this is possible in a scatter plot?