knot points at specific points in the chart

How do I mark knot points at specific points on the curve not all. For example, I plot the following curve - (开发者_如何学Gox1,y1) (x2,y2) (x3,y3) (x4,y4) (x5,y5)

I want to mark knot points on x2 and x4. Is there some way to do this?

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Make two data series, one with all points, another with the points you want marked. Format the first (all points) to display the line without markers, and the second (selected points) to display markers without the line.