Getting Your Data into Shape - R Graphics - page 453

Figure 15-3. Left: bar graph with a missing combination; right: with missing combination filled

To fill in the missing combination ( Figure 15-3 , right), give ddply() the .drop=FALSE flag:

c2b <- ddply(c2, c( "Cult" , "Date" ), . drop = FALSE FALSE , summarise,

Weight = mean(HeadWt, na.rm = TRUE TRUE ),

sd = sd(HeadWt, na.rm = TRUE TRUE ),

n = sum(!is.na(HeadWt)))

c2b

Cult Date Weight sd n
c39 d16 3.255556 0.9824855 9
c39 d20 2.722222 0.1394433 9
c39 d21 2.740000 0.9834181 10
c52 d16 2.260000 0.4452215 10
c52 d20 3.044444 0.8094923 9
c52 d21 NaN NA 0

# Make the graph
ggplot(c2b, aes(x = Date, fill = Cult, y = Weight)) + geom_bar(position = "dodge" )

See Also
If you want to calculate standard error and confidence intervals, see Summarizing Data with
Standard Errors and Conidence Intervals .

See Adding Means to a Box Plot for an example of using stat_summary() to calculate means
and overlay them on a graph.
To perform transformations on data by groups, see Transforming Variables by Group .