title: ggplot geom_tile overlay plot with points
body: <pre><code>severity &lt;- c("Major","Serious","Minor","Negligible")
  probability &lt;- c("Highly Probable","Probable","Possible","Remote","Unlikely","Impossible")
  df &lt;- expand.grid(x=severity,y=probability)
  df$x &lt;- factor(df$x, levels=rev(unique(df$x)))
  df$y &lt;- factor(df$y, levels=rev(unique(df$y)))
  df$color &lt;- c(1,1,2,2,1,2,2,2,2,2,2,3,2,2,3,3,2,3,3,3,3,3,3,3)
  
  ggplot(df,aes(x,y,fill=factor(color)))+
  geom_tile(color="black")+
  scale_fill_manual(guide="none",values=c("red","yellow","green"))+
  scale_x_discrete(expand=c(0,0))+scale_y_discrete(expand=c(0,0))+
  labs(x="",y="")
  </code></pre>
  
  <p>Produces a risk assesssment score card chart.  I want to add points by using a csv file by adding a record.  Each record has 3 fields, a item name, x, and y coordinate. x= severity and y = probability.  </p>
  
  <pre><code>   da &lt;- data.frame(list(name=c("ENVIRONMENTAL","COSTS","SUPPLY","HEALTH"),
                      severity=c("Major","Serious","Minor","Serious"),
                      probability=c("Probable","Possible","Probable","Unlikely")))
  da
             name severity probability
  1 ENVIRONMENTAL    Major    Probable
  2         COSTS  Serious    Possible
  3        SUPPLY    Minor    Probable
  4        HEALTH  Serious    Unlikely
  
  &gt; p1 &lt;- p + data.frame(da, aes(severity, probability)) + geom_point()
  Error in as.data.frame.default(x[[i]], optional = TRUE, stringsAsFactors = stringsAsFactors) : 
    cannot coerce class ""uneval"" to a data.frame
  &gt; 
  
  &gt; d &lt;- data.frame(list(name=c("ENVIRONMENTAL","COSTS","SUPPLY","HEALTH"),
      severity=c(2,3,4,1),probability=c(3,5,4,6)))
  &gt; d
             name severity probability
  1 ENVIRONMENTAL        2           3
  2         COSTS        3           5
  3        SUPPLY        4           4
  4        HEALTH        1           6
  &gt; ggplot(d,x=severity, y=probability)+ geom_point()
  Error in exists(name, envir = env, mode = mode) : 
    argument "env" is missing, with no default
  </code></pre>
  
  <p>How can I add points to the ggplot / geom_tile graph?</p>