Plot a histogram based on the Date class - r

Question

I was looking around the web but could not find the answer that I' looking for.
Here is my input data:
Date Calls
2012-01-01 3
2012-01-01 3
2012-01-01 10
2012-03-02 15
2012-03-02 7
2012-03-02 5
2012-04-02 0
2012-04-02 5
2012-04-02 18
2012-04-02 1
2012-04-02 0
2012-05-02 2
I want to plot a hist that will have the sum of calls for each of the days in the "Date" column.
Yes, it can be done by identifying the levels of Date column and add up the corresponding Calls but wondering
if as an elegant way to do it. "Date" column is as "Date" data.class().
According to this example, the final hist should have 4 bins of (16, 27, 24, 2).
Cheers,

Answer 1

Well, technically a histogram is really only to estimate the density function of continuous data and the way you have your data coded, it's more like Date is a categorical variable. So you probably just want a bar chart of counts than a true histogram. You can do what with ggplot with
qplot(Date,Calls, data=dd, stat="summary", fun.y="sum", geom="bar")

Answer 2

Read data:
d <- read.table(text=
"Date Calls
2012-01-01 3
2012-01-01 3
2012-01-01 10
2012-03-02 15
2012-03-02 7
2012-03-02 5
2012-04-02 0
2012-04-02 5
2012-04-02 18
2012-04-02 1
2012-04-02 0
2012-05-02 2",
header=TRUE)
d$Date <- as.Date(d$Date)
library(plyr)
s <- ddply(d,"Date",summarize,Calls=sum(Calls))
library(ggplot2)
If we use Date as the x variable we get month labels:
ggplot(s,aes(x=Date,y=Calls))+geom_bar(stat="identity")
You might prefer the particular date labels:
ggplot(s,aes(x=factor(Date),y=Calls))+geom_bar(stat="identity")
Or non-default labels:
ggplot(s,aes(x=format(Date,"%d-%b"),y=Calls))+geom_bar(stat="identity")+
labs(x="Date")
It should also be possible to do this by constructing your own hist object and passing it to plot.histogram, but I think this way is easier ...