add lab.NewPlotFrom helper function

docs/content/compression-tutorial.md

@@ -77,7 +77,6 @@ From **a2**, we can compute and plot **x2**, the approximation of **x** based on
 
 ```Goal
 # x2 := U @ a2
-plt := lab.NewPlot(b)
-*plt = *fig1
+plt := lab.NewPlotFrom(fig1, b)
 plt.Add(plots.NewLine(plot.NewY(x2)))
 ```

lab/plot.go

@@ -12,7 +12,7 @@ import (
 
 // NewPlot is a simple helper function that does [plot.New] and [plotcore.NewPlot],
 // only returning the [plot.Plot] for convenient use in lab plots. See [NewPlotWidget]
-// for a version that also returns the [plotcore.Plot].
+// for a version that also returns the [plotcore.Plot]. See also [NewPlotFrom].
 func NewPlot(parent ...tree.Node) *plot.Plot {
 	plt, _ := NewPlotWidget(parent...)
 	return plt
@@ -26,3 +26,10 @@ func NewPlotWidget(parent ...tree.Node) (*plot.Plot, *plotcore.Plot) {
 	pw := plotcore.NewPlot(parent...).SetPlot(plt)
 	return plt, pw
 }
+
+// NewPlotFrom is a version of [NewPlot] that copies plot data from the given starting plot.
+func NewPlotFrom(from *plot.Plot, parent ...tree.Node) *plot.Plot {
+	plt := NewPlot(parent...)
+	*plt = *from
+	return plt
+}