title: Error in adding annotations to independent facets
body: <p>plot stomatal conductance among different CO2, species and days.</p>
  
  <pre><code>p&lt;-ggplot(data=gssummary,aes(x=factor(day),y=Cond,fill=factor(co2)))+geom_bar(stat = "identity",position = "dodge",width=0.4,colour="black")+geom_errorbar(aes(ymin=Cond-ci,ymax=Cond+ci), width=.1,position = position_dodge(0.4))+theme_bw()+theme(panel.grid=element_blank())+labs(x="Day",y=expression(plain("g"["s"])))+facet_grid(~species)+theme(axis.ticks=element_line(size=rel(0.1)))+theme(axis.text=element_text(size = rel(0.5),family = "Times"))+theme(axis.title=element_text(size = 7,family = "Times"))+theme(plot.margin=unit(c(0,3,0,0),"cm"))+theme(strip.text=element_text(size=rel(0.6),family = "Times"),strip.background=element_rect(colour = "white",fill = "white"))+theme(panel.border=element_rect(colour = "black",size = 1))+guides(fill=guide_legend(title=NULL))+scale_fill_grey(start = 1,end = 0.75,labels=c(expression(plain("aCO"["2"])),expression(plain("eCO"["2"]))))
  </code></pre>
  
  <p>add the information of significance to the plot(The difference was only significant between aco2 and eco2 on 2nd Sep,2016 in species 0)</p>
  
  <pre><code>siglabel&lt;-data.frame(species=c("0","1"),label=c("*"," "))
  p+geom_text(x=1,y=0.6,aes(label=label),data = siglabel)
  </code></pre>
  
  <p><strong>Error: Aesthetics must either be length one, or the same length as the dataProblems:label</strong></p>
  
  <p><a href="https://i.stack.imgur.com/T5ZeC.jpg" rel="nofollow">stomatal conductance</a></p>
  
  <p><a href="https://i.stack.imgur.com/NEFAk.jpg" rel="nofollow">gssummary</a></p>
  
  <p>I've tried but still can not solve this problem. Please tell me how to solve it. Thanks a lot!</p>