geom_blank drops NA : <p>Using <code>geom_blank</code> I want to add some new factor levels, but I can't seem to do this <em>and</em> keep the <code>NA</code> level</p>

<pre><code>library('ggplot2')
pl &lt;- ggplot(data.frame(x = factor(c(1:2, NA)), y = 1), aes(x, y)) + geom_point()
pl
</code></pre>

<p><a href="https://i.stack.imgur.com/DrAES.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/DrAES.png" alt="enter image description here"></a></p>

<pre><code>pl + geom_blank(data = data.frame(x = addNA(factor(c(0:3, NA))), y = 1))
</code></pre>

<p><a href="https://i.stack.imgur.com/eFlWE.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/eFlWE.png" alt="enter image description here"></a></p>

<p>I would like to have the x at 0,1,2,3,NA using <code>geom_blank</code></p>
