[R] identifying groups in xyplot

[wisc_maier]

There have been posts in the past regarding similar questions, but many of
them looked dated. I am using xyplot to show variability within my
replicates, and so far so good, but I would like to refine the plot. I've
used the code below to graph my response variable against year (coded as a
factor with three levels 2009, 2010, 2011). Each replicate is in a different
panel (|field). My group variable is a factor (Management) with three
levels. I would like to define the symbol and color of each factor level. 

xyplot(Hill.s.diversity ~ Year| Field, group=Management,
layout=c(2,3),
main="Hills evenness by Management Block, June 2009-2011",
ylab="Hills evenness",
data=summer_pr_avg)

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