You cannot reference the ViewPort (winChartViewer1) from the XYChart. You can only reference the XYChart from the winChartViewer1 (by using the winChartViewer1.Chart property) The XYChart is the input to the winChartViewer1. You can consider the winChartViewer1 to be a container of the XYChart, analogous to an ArrayList being a container for other elements. You can access the elements from the container, but not the reverse.
If you want to display the chart, at some point your code will need to assign the chart to the winChartViewer1 for display. That means your code already have the reference to the winChartViewer1 and hence the viewport.