Build Graph(Plot) in R: median prices through time intervals

Question

I have the data frame with prices and the ending date of some auctions. I want to check when appears, for example, sales with minimal and maximal prices (also the median) depending on the time of the day.

More precisely, I have the data frame mtest :

> str(mtest)
'data.frame':   9144 obs. of  2 variables:
$ Price      : num  178 188 228 305 202 ...
$ EndDateTime: POSIXct, format: "2015-05-25 05:00:59" "2015-05-23 00:06:01"  ...

I want to build the graph(plot), having 30 minutes time internals (00:00-00:30, 00:31-01:00 etc) on the X axis, and median (maximal, minimal prices) on Y axis.
Another idea is to draw a simple histogram for each time interval, like hist(mtest$Price, breaks=10, col="red")

How can I do this in the best way?

Answer 1

Try this:

cutt=seq(from=min(mtest$EndDateTime),to=max(mtest$EndDateTime), by=30*60)
if (max(mtest$EndDateTime)>max(cutt))cutt[length(cutt)+1]=max(cutt)+30*60
mtest$tint=cut(mtest$EndDateTime,cutt)
stats=do.call(rbind,tapply(mtest[,"Price"],mtest[,"tint"],
                     function(p)c(min=min(p),median=median(p),max=max(p))))
bp=boxplot(mtest[,"Price"]~mtest[,"tint"],xaxt="n",
           col=1:length(levels(mtest$tint)))
axis(1,at=1:length(levels(mtest$tint)),labels=format.Date(levels(mtest$tint),"%Y-%m-%d %H:%M"),
     las=2,cex.axis=.5)
stats

Or wilth plot

plot(NA,ylim=range(stats),xlim=c(1,lint),type="n",xaxt="n",xlab="",ylab="")
sapply(1:3,function(z)points(stats[,z]~c(1:lint),col=z))
axis(1,at=1:lint,labels=format.Date(levels(mtest$tint),"%Y-%m-%d %H:%M"),
     las=2,cex.axis=.5)

You will have something like this: