Fixed a bug which caused the program to fail when attempting to remove
O2 graph whithout selecting it.
improved hideCommentTracer so it returns safely (not crash) if there are
no comment tracers.
added code for rescale button.
fixed rescaling of graphs.
added toggling of the context menu for when a selected graph is removed.
added remove tracer functionality when removing a selected graph.
added remove tracer functionality when all graphs are removed.