Re: [R] ylim for graphic

From: Marc Schwartz (via MN) <mschwartz_at_mn.rr.com>
Date: Tue 30 Aug 2005 - 03:35:38 EST

On Mon, 2005-08-29 at 12:58 -0400, Doran, Harold wrote:
> Dear list:
>
> I have some data for which I am generating a series of barplots for
> percentages. One issue that I am dealing with is that I am trying to get
> the legend to print in a fixed location for each chart generated by the
> data. Because these charts are being created in a loop, with different
> data, my code searches the data to identify the maximum value in the
> data and then print the data values 5 points above the tallest bar.
>
> Now, in situations where the largest value is 100, I needed to create
> the y-axis high enough to accommodate the legend w/o crowding up the
> data or the bars. So, I have ylim =c(0,200) and then place the legend at
> c(2,150). Visually, this places things exactly where I want them. But,
> it seems silly to have a y-axis labeled as high as 200%. I'm certain
> there is a smarter technique. Is it possible to place the legend at a
> location higher than 100 to avoid the crowding of the bars and the data
> and then also have the labels for the y-axis not print after the value
> 100%?
>
> In looking at ?legend I didn't see an option that would address this.
> Below is some code that you can use to create a similar chart.
>
> Thanks,
> Harold
>
>
> math.bar <- c(53,31,55,28,55,100)
> apmxpmeet <- c(47, 50, 49, 50, 49, 46)
> par(ps=10)
> math.bar <- rbind(math.bar, apmxpmeet)
> math.barplot <- barplot(math.bar, beside=T, col=c('blue','orange'),
> names=c('Grade \n 3','Grade \n 4','Grade \n 5','Grade \n 6','Grade \n
> 7','Grade \n 8'),
> ylim=c(0,200), ylab="Percentage", xlab="Grade Level")
> tot <- round(math.bar,digits=0)
> graph.max <- max(math.bar, apmxpmeet, na.rm=T)
> text(math.barplot, graph.max+5, tot, xpd = TRUE, col = c("blue",
> "orange") )
> legend(2,150,legend=(c("Label A", "Average")), fill=c("blue","orange"))

Harold,

A few thoughts:

  1. Instead of fixing the y axis max value at 200, simply set ylim to c(0, max(math.bar * 1.2)) or a similar constant. In this case, you get an extra 20% above the max(y) value for the legend placement.
  2. In the legend call, use:

  legend("topleft", legend=(c("Label A", "Average")),

          fill = c("blue","orange"))

This will place the legend at the topleft of the plot region, rather than you having to calculate the x and y coords. If you want it moved in from the upper left hand corner, you can use the 'inset' argument as well, which moves the legend by a proportion of the plot region limits (0 - 1):

    legend("topleft", legend=(c("Label A", "Average")),

          fill = c("blue","orange"), inset = .1)

3. You can use barplot(..., yaxt = "n") to have the y axis not drawn and then use axis() to place the labels at locations of your choosing, which do not need to run the full length of the axis range:

 barplot(1:5, yaxt = "n", ylim = c(0, 10))  axis(2, at = 0:5)

4. You can place the legend outside the plot region, enabling you to keep the y axis range to <=100. This would need some tweaking, but the idea is the same:

 # Increase the size of the top margin
 par(mar = c(5, 4, 8, 2) + 0.1)

 # Draw a barplot
 barplot(1:5)

 # Disable clipping outside the plot region  par(xpd = TRUE)

 # Now draw the legend, but move it up by 30% from the top left  legend("topleft", legend = LETTERS[1:5], inset = c(0, -.3))

You could also place the legend to the right or left of the plot region if you prefer, adjusting the above accordingly.

HTH, Marc Schwartz



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