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POWhy does geom_text() throw coercion errors when hjust and vjust are strings?
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copied!<p>I've noticed an unexpected behavior in <code>ggplot2</code>'s <code>geom_text()</code> geom. If the attributes <code>hjust</code> and <code>vjust</code> are specified as strings, R returns coercion errors, though the plots seem to come out OK. The problem came up in a <a href="https://github.com/briandk/granova/tree/dev" rel="nofollow">ggplot2-based package</a> I'm developing. For simplicity, I've created stripped-down examples that still produce the error.</p> <p>First, I tried it with <code>qplot()</code></p> <pre><code>##qplot version library(ggplot2) p <- qplot(cty, hwy, label = drv, hjust = "right", geom = "text", data = mpg ) print(p) </code></pre> <p>And I got this error:</p> <pre><code>Warning message: In validDetails.text(x) : NAs introduced by coercion </code></pre> <p>Then I tried it with <code>ggplot()</code>: </p> <pre><code>##ggplot version library(ggplot2) p <- ggplot( aes(x = cty, y = hwy ), data = mpg ) p <- p + geom_text( aes(label = drv), hjust = "right" ) print(p) </code></pre> <p>and got an identical plot, and an identical error:</p> <pre><code>Warning message: In validDetails.text(x) : NAs introduced by coercion </code></pre> <p>I then tried setting both hjust and vjust:</p> <pre><code>library(ggplot2) p <- ggplot( aes(x = cty, y = hwy ), data = mpg ) p <- p + geom_text( aes(label = drv), hjust = "right", vjust = "top" ) print(p) </code></pre> <p>With both parameters set using strings, R returns two coercion errors:</p> <pre><code>Warning messages: 1: In validDetails.text(x) : NAs introduced by coercion 2: In validDetails.text(x) : NAs introduced by coercion </code></pre> <p>But, when the parameters are numbers, R returns no coercion errors:</p> <pre><code>## Using numbers instead of strings library(ggplot2) p <- ggplot( aes(x = cty, y = hwy ), data = mpg ) p <- p + geom_text( aes(label = drv), hjust = 0, vjust = 0, data = mpg ) print(p) </code></pre> <p>I'm not quite sure why this happens, or whether it's significant, but I didn't expect it.</p> <h3>ggplot2 documentations don't agree</h3> <p><a href="http://books.google.com/books?id=F_hwtlzPXBcC&printsec=frontcover&dq=ggplot2&hl=en&src=bmrr&ei=DgJjTd3EBMGclgfs6ezhCw&sa=X&oi=book_result&ct=result&resnum=1&ved=0CCcQ6AEwAA" rel="nofollow">Hadley's book</a>(p. 196) says <code>hjust</code> and <code>vjust</code> can accept string arguments:</p> <blockquote> <p>Justification of a string (or legend) defines the location within the string that is placed at the given position. There are two values for horizontal and vertical justification. The values can be:</p> <ul> <li>A string: "left", "right", "centre", "center", "bottom", and "top". </li> <li>A number between 0 and 1, giving the position within the string (from bottom-left corner).</li> </ul> </blockquote> <p>But the man file for <code>geom_text()</code> in version 0.8.9 says hjust and vjust are numeric, though it <em>doesn't</em> say they can <em>only</em> be numeric:</p> <blockquote> <p>Aesthetics</p> <p>The following aesthetics can be used with geom_text. Aesthetics are mapped to variables in the data with the aes function: geom_text(aes(x = var))</p> <ul> <li>x: x position (required)</li> <li>y: y position (required)</li> <li>label: text label (required)</li> <li>colour: border colour</li> <li>size: size</li> <li>angle: angle</li> <li>hjust: horizontal justification, between 0 and 1</li> <li>vjust: vertical justification, between 0 and 1</li> <li>alpha: transparency</li> </ul> </blockquote>

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