dates on xaxis are not readable

Question

I have a data frame that has dates, disk reads and disk writes data. I am able to create the stack chart but the dates on xaxis are not readable. Is there an option in qplot to scale the graph or dont display all of the dates or something? I've been looking at this now for a long time, I couldn't find an answer.

I melt it first

app1_diskIO.M<-melt(app1_diskIO.M, id=c("Date2"))

qplot(x = factor(Date2), y = value, data = app1_diskIO.M, geom = "bar", fill = variable, main="DISK IO", xlab="Date", ylab="Disk IO")

Data frame looks like this:

       Date2 DiskReads DiskWrites
1  2011-12-06  136382.2    78961.0
2  2011-12-07  146277.0    77802.0
3  2011-12-08  141123.6    76219.0
4  2011-12-09  139219.8    77649.4
5  2011-12-10   72101.1    35258.6
6  2011-12-11  229445.9    92316.6
7  2011-12-12  175886.7    86087.0
8  2011-12-13  134958.3    76605.6
9  2011-12-14  133972.1    75141.4
10 2011-12-15  147175.2    77189.7
11 2011-12-16  143542.3    78155.9
12 2011-12-17  152579.1    68395.2
13 2011-12-18  168667.7    69720.1
14 2011-12-19  152328.6    75994.2
15 2011-12-20  135061.8    75271.6
16 2011-12-21  153455.0    78197.5
17 2011-12-22  130020.8    73369.7
18 2011-12-23  131423.6    73484.9
19 2011-12-24  135285.6    65081.4
20 2011-12-25  137185.4    63334.8
21 2011-12-26  132612.9    70484.4
22 2011-12-27  155396.9    79239.1
23 2011-12-28  151587.5    77986.5
24 2011-12-29  122076.5    71575.5
25 2011-12-30  146888.8    75376.3
26 2011-12-31  141285.5    69737.8
27 2012-01-01  206306.6    78059.4
28 2012-01-02  134002.4    73062.5
29 2012-01-03  137753.8    76947.4
30 2012-01-04  150655.8    78836.3
31 2012-01-05  151750.3    79300.9
32 2012-01-06  141464.8    77529.4
33 2012-01-07  137667.3    66344.8
34 2012-01-08  120998.5    60582.4
35 2012-01-09  133422.4    73688.3
36 2012-01-10  134247.7    75664.5
37 2012-01-11  146243.5    79042.2
38 2012-01-12  178738.2    84199.7
39 2012-01-13  145564.8    77908.2
40 2012-01-14  135966.8    65900.9
41 2012-01-15  136448.4    63339.9
42 2012-01-16  186109.9    83348.6
43 2012-01-17  146906.5    78645.6
44 2012-01-18  106343.0    69313.6
45 2012-01-19  197205.6    88411.6
46 2012-01-20  132950.1    73078.0
47 2012-01-21  137488.1    68711.5

Answer 1

How about rotating the dates to be vertical, that makes them a bit more legible:

qplot(x = factor(Date2), y = value, data = app1_diskIO.M, 
     geom = "bar", fill = variable, main = "DISK IO", 
     xlab = "Date", ylab = "Disk IO") +
     opts(axis.text.x = theme_text(angl e =-90))

You could also reduce the font size (is that what you mean by scale?), but that doesn't make it any easier to read. To do this, add a size argument in the opts

qplot(x = factor(Date2), y = value, data = app1_diskIO.M, 
     geom = "bar", fill = variable, main = "DISK IO", 
     xlab = "Date", ylab = "Disk IO") +
     opts(axis.text.x = theme_text(angle = -90, size = 5))

To remove all tick marks and labels on X axis (so you're not displaying any dates, as you request, maybe you might use a graphics editor to put something else there?), try this:

qplot(x = factor(Date2), y = value, data = app1_diskIO.M, 
     geom = "bar", fill = variable, main = "DISK IO", 
     xlab = "Date", ylab = "Disk IO")  + 
     opts(axis.ticks = theme_blank(), axis.text.x = theme_blank())

UPDATE: To space out the x axis labels, say one every ten days, as you request, try this:

app1_diskIO.M$Date2 <- as.Date(app1_diskIO.M$Date2)
qplot(x = Date2, 
      y = value, data = app1_diskIO.M, 
      geom = "line", colour = variable, 
      main="DISK IO", xlab="Date", 
      ylab="Disk IO") +
      scale_x_date(breaks = "10 days") +
      opts(axis.text.x = theme_text(angle = -90))

Which should give you:

Useful website with excellent instructions for doing this kind of stuff: http://wiki.stdout.org/rcookbook/Graphs/Axes%20(ggplot2)/

The guide to the updates in ggplot2 0.9 is very handy too, with lots of good examples: http://cloud.github.com/downloads/hadley/ggplot2/guide-col.pdf

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