Add a graph to the graphswitch widget using a loongraph object

Loongraphs can be created with the loongraph function.

# S3 method for loongraph
l_graphswitch_add(widget, graph, label = "", index = "end", ...)

Arguments

widget	graphswitch widget handle (or widget path)
graph	a loongraph object
label	string with label for graph
index	position of graph in the graph list
...	additional arguments are not used for this methiod

id for graph in the graphswitch widget