When displaying a line style chart and using a legend with a legendMarkerType set, the legend marker reverts to a simple line if the number of points rendered causes the markers on the actual chart line to disappear.
You can see this in action by adding legendMarkerType: “square”, showInLegend:true to your Basic Line Example
legendMarkerType falls back to markerType set in the dataSeries. Setting markerType / legendMarkerType to ‘none’ would be the reason in this scenario. Can you kindly create jsfiddle with sample data, so that we can understand your code better and help you out?