Give the user more control about which curves to plot. Default to the two final curves and add an argument controlling wether the toxicant and tox+env curves should be drawn. Keep the argument for the simple models. So there will be three booleans for three pairs of curves.
Also accept the ... argument for more fine control of axes, colors, pty, etc. Remember to use hasArg().
The axes look bad when saving the plots with type = "cairo". This comes from anti-aliasing when overlaying multiple lines.