title: Simmer plot resource usage by attributes
body: <p>Is is possible to plot resource usage by attribute in simmer?  So for example in this simulation:</p>
  
  <pre><code>library(simmer)
  library(simmer.plot)
  
  workerCount &lt;- 2
  
  actualData &lt;- data.frame(
    time = c(1:10,1:5), priority = 1:3, service = rnorm(150, 50, 5)) %&gt;%
    dplyr::arrange(time)
  
  actualData$gender&lt;-floor(runif(150, min=1, max=3))
  
  actualData
  
  activityTraj &lt;- trajectory() %&gt;%
    seize('worker') %&gt;%
    timeout_from_attribute("service") %&gt;%
    release('worker')
  
  env &lt;- simmer() %&gt;%
    add_resource('worker', workerCount, Inf, preemptive = TRUE) %&gt;%
    add_dataframe('worker_', activityTraj, actualData, mon=2, col_time="time", time="absolute", col_attributes=c("gender","service")) %&gt;%
    run()
  
  
  plot(get_mon_attributes(env), keys="gender", metric="usage")
  </code></pre>
  
  <p>The plot generated shows gender over time but not resource usage. The documentation for plot.attributes says:</p>
  
  <blockquote>
    <p>The S3 method for 'attributes' does not support any metric. It simply shows a stairstep graph of the values throughout the simulation for the keys provided (or all the collected attributes if no key is provided).</p>
  </blockquote>
  
  <p>Is there any way currently to plot resource usage by attributes?</p>
  
  <p>Thanks for any suggestions.</p>