[R] add error bars in a plot

[Ben Bolker]

> > I have a dataset of how metal concentrations change through time. I have 
> > made a plot of date versus metal concentration. However I want to add 
> > error bars in the plot.
> >

<http://cran.r-project.org/doc/FAQ/R-FAQ.html#How-can-I-put-error-bars-or-confidence-bands-on-my-plot_003f>

  cheers
     Ben Bolker