If you're creating a scientific graphic in the R language, there's a good chance you'll be wanting to include some mathematical symbols somewhere on the chart. You might want to use a symbol like μ as an axis label, annotate a curve with simple math like x2, or even put a complete equation like:
in the title. You can do this easily in R using the plotmath syntax. Using escape sequences reminiscent of LaTeX, you can include beautifully-formatted mathematical symbols and equations in charts wherever you can place text. For example, the summation above can be added to a plot with an R statement like text(2,5,expression(sum(x[i], i=1, n)). This cheat sheet on mathematical annotation in R provides a guide to commonly-used symbols and equations, including:
The cheat sheet comes courtesy of Vistat, a site that provides working recipes for statistical graphics. R programmers should also check out this cheat sheet for plotting symbols and colors for R graphics.