Unintended line connecting two data points
 

Greetings,

I have a chart with 13 data points.
No trendline is in use.

A black line connecting the 8th and 9th data points has mysteriously appeared, and I cannot get rid of it. I've tried reformatting everything from the data points to both axes. I've erased and re-entered the source data. Nothing.

What is it?
Why is it there?
How can I get rid of it?

Pleaseandthankyouverymuch!
E