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POHow can I overlay by-group plot elements to ggplot2 facets?
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copied!<p>My question has to do with facetting. In my example code below, I look at some facetted scatterplots, then try to overlay information (in this case, mean lines) on a per-facet basis.</p> <p>The tl;dr version is that my attempts fail. Either my added mean lines compute across all data (disrespecting the facet variable), or I try to write a formula and R throws an error, followed by incisive and particularly disparaging comments about my mother.</p> <pre><code>library(ggplot2) # Let's pretend we're exploring the relationship between a car's weight and its # horsepower, using some sample data p <- ggplot() p <- p + geom_point(aes(x = wt, y = hp), data = mtcars) print(p) # Hmm. A quick check of the data reveals that car weights can differ wildly, by almost # a thousand pounds. head(mtcars) # Does the difference matter? It might, especially if most 8-cylinder cars are heavy, # and most 4-cylinder cars are light. ColorBrewer to the rescue! p <- p + aes(color = factor(cyl)) p <- p + scale_color_brewer(pal = "Set1") print(p) # At this point, what would be great is if we could more strongly visually separate # the cars out by their engine blocks. p <- p + facet_grid(~ cyl) print(p) # Ah! Now we can see (given the fixed scales) that the 4-cylinder cars flock to the # left on weight measures, while the 8-cylinder cars flock right. But you know what # would be REALLY awesome? If we could visually compare the means of the car groups. p.with.means <- p + geom_hline( aes(yintercept = mean(hp)), data = mtcars ) print(p.with.means) # Wait, that's not right. That's not right at all. The green (8-cylinder) cars are all above the # average for their group. Are they somehow made in an auto plant in Lake Wobegon, MN? Obviously, # I meant to draw mean lines factored by GROUP. Except also obviously, since the code below will # print an error, I don't know how. p.with.non.lake.wobegon.means <- p + geom_hline( aes(yintercept = mean(hp) ~ cyl), data = mtcars ) print(p.with.non.lake.wobegon.means) </code></pre> <p>There <em>must</em> be some simple solution I'm missing.</p>

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