After you have manually picked points on a curve, or you have automatically traced points on a curve and added some points by hand, you may want to sort your data points.
You can sort your all data points in your current data set in increasing X direction by clicking on the sort button next to the data point list in the top of the window. You can also press Ctrl+T.
All other sort directions you can find from the Sort Current Data Set submenu of the Data menu.
See also: Manipulating data.