plotutils.AddLines stores names in a map which causes legend ordering issues

Created by: skiesel

https://github.com/gonum/plot/blob/master/plotutil/add.go#L179

When calling plotutils.AddLines the names passed in get stored temporarily in a map which causes the legend ordering to get mixed up. When creating multiple plots it can be important to have the lines/glyphs matched up and in the same order across all plots. If this was a slice instead of a map the order would be preserved.