I assume it is good to post solutions to problems we solve so others can see them if they search this forum.
I had a spline (and tried a line) that was only showing the marker, not the actual line, in conjunction with a column series. The spline was a running 3-month average following the columns, but I was only seeing the circle marker, no (sp)line.
Since it was a 3-month running average, the last value was not getting set and the spline data array was one shorter than the column data array. This resulted in only getting the marker. When I filled the spline series data array with a final value, the line showed up.