The reason this is happening is that a sparkline can only draw a line between two data points if there are two data points in a row to draw a line between.
In the case of row 15, there is an empty space and then only a single item of data, meaning there is no end point for a line to be drawn to.
The solution is to enable Markers for your sparklines. This means that markers will appear for each value instead of lines alone, and will allow you to see the lone data point in row 15. The default markers can be a little hard to see, so you may want to customize them to be larger or to appear in a different color.
You can see a complete walkthrough of adding markers to sparklines in the video lesson: Essential Skills Lesson 4-20 Apply a common vertical axis and formatting to a Sparkline group.
I have also attached a copy of your workbook with the markers applied:
sparkline problem.xlsx 205.99KB
I hope this is the solution you were looking for, but please feel free to reply if you need any more help with this.
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