Plotrix in Grid Extra: "no grobs' allowed in "gList"
<p>I created a battleship plot using Plotrix:</p>

<pre><code>k&lt;-battleship.plot(data, main="Distribution",xlab="Type",ylab="Phase", 
                                                        maxxspan=0.6,maxyspan=0.45)

j&lt;-battleship.plot(data2, main="Distribution",xlab="Type",ylab="Phase", 
                                                        maxxspan=0.6,maxyspan=0.45)
</code></pre>

<p>I want to print both plots together using gridExtra:</p>

<pre><code>library(gridExtra)
grid.arrange(j, k, ncol=1, nrow =2, top=("Types by Phase"))
</code></pre>

<p>I get the following error:</p>

<pre><code>Error in gList(list(mar = c(2, 5, 5, 1), wrapvp = list(x = 0.5, y = 0.5,  : 
  only 'grobs' allowed in "gList"
</code></pre>

<p>I have tried arrangeGrob without success.</p>

<p>How can overcome this problem?
ggplot2 plots are working great, even with the same data.</p>