title: Plot timeseries as heatmap
body: <p>I have a data frame with timestamps and values (a month with values approximately every 5min).
  Now I want to have plot with the day on the x-axis, the time of day on the y-axis and the value as the color. Best would be if I can control the binning on the y-axis (5min, 10min, 15min.. with the value as the mean for each bin).</p>
  
  <p>I am experimenting with ggplot2, but I can't get a reasonable result.</p>
  
  <pre><code>p_heat &lt;- ggplot(data = data,
                   aes(x = days(timestamp),
                       y = minutes(timestamp) + hours(timestamp) * 60, fill = value)) +  
              geom_tile()
  </code></pre>
  
  <p>Test data can be generated as follows:</p>
  
  <pre><code>data &lt;- data.frame(timestamp = seq(from = as.POSIXct("2013-09-01 00:00:00"), 
                                     to = as.POSIXct("2013-10-01 00:00:00"), by = "5 mins"),
                     value = runif(8641, 0, 1))
  </code></pre>
  
  <p>This was the best I could get so far :)
  I also tried playing with scale_x/y_date()</p>
  
  <p>I would be glad about some hints which push me into the right direction. Thanks!</p>