amarillion - 1 year ago 87

R Question

I have a bunch of measurements over time and I want to plot them in R. Here is a sample of my data. I've got 6 measurements for each of 4 time points:

`values <- c (1012.0, 1644.9, 837.0, 1200.9, 1652.0, 981.5,`

2236.9, 1697.5, 2087.7, 1500.8,

2789.3, 1502.9, 2051.3, 3070.7, 3105.4,

2692.5, 1488.5, 1978.1, 1925.4, 1524.3,

2772.0, 1355.3, 2632.4, 2600.1)

time <- factor (rep (c(0, 12, 24, 72), c(6, 6, 6, 6)))

The scale of these data is arbitrary, and in fact I'm going to normalize it so that the average of t=0 is 1.

`norm <- values / mean (values[time == 0])`

So far so good. Using

`ggplot`

`require (ggplot2)`

p <- ggplot(data = data.frame(time, norm), mapping = aes (x = time, y = norm)) +

stat_summary (fun.y = mean, geom="line", mapping = aes (group = 1)) +

geom_point()

However, now I want to apply a logarithmic scale, and this is where my trouble starts. When I do:

`q <- ggplot(data = data.frame(time, norm), mapping = aes (x = time, y = norm)) +`

stat_summary (fun.y = mean, geom="line", mapping = aes (group = 1)) +

geom_point() +

scale_y_log2()

The line does NOT go through 0 at t=0, as you would expect because log (1) == 0. Instead the line crosses the y-axis slightly below 0. Apparently,

`ggplot`

How can I tell

`ggplot`

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Answer Source

`scale_y_log2()`

will do the transformation first and then calculate the geoms.

`coord_trans()`

will do the opposite: calculate the geoms first, and the transform the axis.

So you need `coord_trans(ytrans = "log2")`

instead of `scale_y_log2()`

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