[R] abline plots at wrong abscissae after boxplot

From: Brian Wilfley <bwilfley_at_tripleringtech.com>
Date: Thu, 21 Jun 2007 14:44:17 -0700

Hi folks,

I'm using R 2.5.0 under ESS under Windows XP. (This also happens using the Rgui application.)

I'm trying to add lines to a plot originally made with "boxplot", but the lines appear in the wrong place. Below is a script that illustrates the problem

# boxablinetest.R - script to show problem with abline on box plot

x <- c( 2, 2, 2, 3, 3, 3, 4, 4, 4) y <- c( 1, 2, 3, 2, 3, 4, 3, 4, 5)

xymodel <- lm( y~x)

boxplot( y~x)

abline( xymodel)                        # Wrong abcissae
abline( v = 2.5)                        # Wrong abcissa
abline( h = 2.75)                       # Right ordinate

# -------------- end --------------

Here, I'm making a box plot with abscissae that start at 2. The box plot looks fine: the numbers 2, 3, and 4 appear on the x-axis and the boxes are centered at 2, 3, and 4.

When I add the first abline, the line appears too low, but actually it is too far to the right. The abscissae are being interpreted without realizing that the plot originates at 2, not 1.

The second call to abline should put a vertical line between 2 and 3, but instead it shows up between 3 and 4. Again, it appears that the offset in the origin of the boxplot is not accounted for.

Finally the last abline appears where it should: between 2 and 3. Evidently, ordinate values are correctly interpreted.

Does anyone have any advice?

Thanks very much.

Brian Wilfley

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