title: S4 Object Issue with scale_shape_manual
body: <p>I am struggling with an issue with scale_shape_manual and scale_colour_manual from ggplot. Retaking the example from ggplot help(scale_colour_manual) we get the following two lines : </p>
  
  <pre><code>p &lt;- qplot(mpg, wt, data = mtcars, colour = factor(cyl))
  p + scale_colour_manual(values = c("red","blue", "green"))
  </code></pre>
  
  <p>They work fine and I can do whatever I want after that. But if I decide to create a line like this one, which would create an object combining the layers</p>
  
  <pre><code>h &lt;- p + scale_colour_manual(values = c("red","blue", "green"))
  </code></pre>
  
  <p>Then any command will prompt me an warning message "Setting class(x) to multiple strings ("manual", "discrete", ...); result will no longer be an S4 object". Even a very simple one : </p>
  
  <pre><code> a = 2
  </code></pre>
  
  <p>However If I recompile the last line it works. Anyone had the same issue ? Can you help me on that ?</p>
  
  <p>Following comment here is a reproducible example</p>
  
  <pre><code>library(ggplot2)
  library(Hmisc)
  data(mpg)
  p &lt;- qplot(mpg, wt, data = mtcars, colour = factor(cyl))
  h  &lt;-  p + scale_colour_manual(values = c("red","blue", "green"))
  a = 2
  </code></pre>
  
  <p>UPDATE - 
  Apparently it was just a version issue, the following version appear not to work together :  R version 3.0.3 (2014-03-06) Hmisc_3.14-3 and ggplot2_0.9.3.1. One might consider detach(package:Hmisc, unload = TRUE) or just accept the warning. </p>
  
  <p>Thanks
  R</p>