title: Gadfly.jl : How to plot date time based?
body: <p>I'm trying to plot data based on time :</p>
  
  <pre><code>userId = 2
  dateGiven = Datetime.date(2014,2,3)
  biometric = "heart-rate"
  df = DataFrames.readtable(string("data/user_",userId,"/",dateGiven,"/",biometric,".csv"),
                        colnames = ["Time", "Heart Rate"],
                        coltypes = {Int,Int}  )
  
  df["Time"] = map((timestamp) -&gt; Datetime.unix2datetime(timestamp, Datetime.UTC) , df["Time"])
  plot(df,x="Time",y="Heart Rate",Geom.line)
  </code></pre>
  
  <p>I have this error :</p>
  
  <pre><code>no method *(Float64,DateTime{ISOCalendar,Zone0})
   in optimize_ticks at /Users/nhenin/.julia/v0.2/Gadfly/src/ticks.jl:77
   in apply_statistic at /Users/nhenin/.julia/v0.2/Gadfly/src/statistics.jl:584
   in apply_statistics at /Users/nhenin/.julia/v0.2/Gadfly/src/statistics.jl:35
   in render at /Users/nhenin/.julia/v0.2/Gadfly/src/Gadfly.jl:636
   in writemime at /Users/nhenin/.julia/v0.2/Gadfly/src/Gadfly.jl:738
   in sprint at io.jl:434
   in display_dict at /Users/nhenin/.julia/v0.2/IJulia/src/execute_request.jl:35
  </code></pre>
  
  <p>Any hints ? </p>