title: non-numeric argument to binary operator in ggplot
body: <p>This is driving me crazy... Sorry if it is trivial. I Searched but did not find help. First of all a reproducible example</p>
  
  <pre><code>df = data.frame(idx = c(1,2,3,4,5), label=c(0,0,1,1,0), 
   filter = c(1,1,0,1,1), values = c(0.1, 0.3, 1.2, 1.7, 2.4))
  
  
  ggplot (df, aes(x = df$idx, y = df$values, size = as.factor(df$label*filter), 
  color = as.factor(df$label*filter)))+geom_point()+
  scale_colour_manual(name = "Duplicates (J/N)", labels = c("Nein", "Ja"), 
  values = c("red", "blue")) + scale_size_manual(name = "Duplicates (J/N)", 
  labels = c("Nein", "Ja"), values = c("red", "blue"))
  </code></pre>
  
  <p>When I run this example I get the error</p>
  
  <pre><code>Error in coords$size * .pt : non-numeric argument to binary Operator
  </code></pre>
  
  <p>Now usually in ggplot you get this error when you got too many + or not enough +. But it does not seem to be the case here. At least is not evident to me.</p>
  
  <p>This is working (without the <code>scale_size_manual</code>)</p>
  
  <pre><code>ggplot (df, aes(x = df$idx, y = df$values, size = as.factor(df$label*filter), 
              color = as.factor(df$label*filter)))+geom_point()+
      scale_colour_manual(name = "Duplicates (J/N)", labels = c("Nein", "Ja"), 
                    values = c("red", "blue")) 
  </code></pre>
  
  <p>Anyone spot the mistake??</p>
  
  <p>Thanks in advance for the help!</p>