@kinokatsu,
The lines drawn at every interval of the axis are gridlines, including the line which you are referring. Gridlines are drawn as dashed lines as you are setting gridDashType to “dash”. It’s not possible to just change dash-type of just one gridline as of now. However you can add a stripline at the maximum of the axis to achieve your requirement. Please take a look at this updated JSFiddle for an example on the same.
—
Vishwas R
Team CanvasJS