Unusual behavior of plots when values of y are small

[Lester Wollman]

The following code gives me a normal looking line plot:

z <- data.frame(x = 1:5, y = 1e-6 * c(1, 1.3, 1.5, -1.2, .8))
ggplot(z, aes(x, y)) + geom_line()

However, if I let y be one order of magnitude smaller,

z <- data.frame(x = 1:5, y = 1e-7 * c(1, 1.3, 1.5, -1.2, .8))
ggplot(z, aes(x, y)) + geom_line()

the plot is a straight line along y. It seems that ggplot assumes
that all numbers below about 1e-6 are effectively the same.

I noticed this only in ggplot2 0.8.9, although I am not sure exactly
when this behavior started. I definitely had normal looking line
plots in earlier versions of ggplot2.

Is this a bug or a feature, and is there a simple way to get normal
looking plots for small values of y without having to multiply all y
values by a constant?

----------------------------

Lester Wollman
Statistician
Symmetricom, Inc.

[Hadley Wickham]

Hi Lester,

Yes, it's a bug, but fortunately it's already fixed in the development
version of ggplot2. The problem arises from my incomplete knowledge
of floating point arithmetic - my previous check for zero range was
too imprecise.

Hadley

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