title: Timeline bar with colour/fill based on time-series' value (R ggplot)
body: <p>I've got a time-series data frame with pressure readings taken at regular intervals.  </p>
  
  <pre><code>                  time   pressure        diff
  1 2014-09-09 09:12:29  1.6191598  0.00000000
  2 2014-09-09 09:12:28  3.0137784 -0.07668387
  3 2014-09-09 09:12:27  1.1958183  0.58693260
  4 2014-09-09 09:12:26  2.2803681  1.07774954
  5 2014-09-09 09:12:25 -0.7614310 -0.17864232
  6 2014-09-09 09:12:24  0.9914106 -0.70121973
  </code></pre>
  
  <p>I can easily make a line plot of the pressure using ggplot2. But below this line plot, I'd like to have a horizontal bar where the fill colour depends on the pressure differential between two consecutive samples (df field <code>diff</code>).<br>
  For example, the bar would be white at times where the pressure differential is zero (i.e. pressure has not changed bewteen two consecutive samples). The fill colour would go towards a deeper shade of (say) red as the differential increases positively, and blue as it increases in the negative values.    </p>
  
  <p>generate sample data:  </p>
  
  <pre><code> dfTimeSeries &lt;- data.frame(time = Sys.time()-seq(1:10), 
                              pressure = rnorm(10,1), 
                              diff = c(0,diff(dfTimeSeries$pressure)))
  </code></pre>
  
  <p>First part of the plot   </p>
  
  <pre><code> ggplot(data = dfTimeSeries)+
       geom_line(aes(x=time, y=pressure))
  </code></pre>
  
  <p>How can I code this horizontal bar that would span along the entire x (time) axis and whose fill colour would vary based on the on the  <code>diff</code> field of my df for the corresponding timestamp?</p>