title: how do you size the plot in knitr pdf output
body: <p>I am trying to use knitr to publish some ggplot output to pdf file.</p>
  
  <p>the command that I am using is this:</p>
  
  <pre><code>```{r, results = 'asis', echo=FALSE, warning=FALSE,tidy=FALSE}
      ggplot(data,aes(datetime, usedmem, group=machine, colour=machine))+geom_line()+
          geom_smooth(method="lm", se=T, colour="blue")+
          facet_wrap(~machine)+theme_bw()
  ```
  </code></pre>
  
  <ol>
  <li><p>is it possible to set the size of the whole plot size. If there are bunch of servers in the data frame, the each plot within facet_wrap shrinks.</p></li>
  <li><p>I see one chart per page, how would I print charts one after anther without wasting any space?</p></li>
  </ol>