For function plots, we often would like to have emphasized zero axes x=0 and y=0.
This can be achieve by adding the expression x*y to the placeholder and setting the line color of that trace to black. This, however, doesn't help outsiders to understand the plot (what is x*y) and requires quite some mouseclicks for the line setting.
If this can't easily be added to the format dialog, then a context menu entry could do the job.
