title: RMarkdown doesnt plot a graph in HTML
body: <p>I've been working on a HTML document with Rmarkdown. </p>
  
  <p>The document has several sp plots and ggplots and all of them appear in the HTML. </p>
  
  <p>But when I call plotK (which is a function from stpp package to plot the spatio-temporal inhomogeneous k-funtion - STIKhat), the plot doesnt appear in the HTML. </p>
  
  <p>Here's a reproducible example for Rmarkdown:</p>
  
  <pre><code>---
  title: "Untitled"
  output: html_document
  ---
  
  ```{r}
  library(stpp)
  data(fmd)
  data(northcumbria)
  FMD&lt;-as.3dpoints(fmd[,1]/1000,fmd[,2]/1000,fmd[,3])
  Northcumbria=northcumbria/1000
  # estimation of the temporal intensity
  Mt&lt;-density(FMD[,3],n=1000)
  mut&lt;-Mt$y[findInterval(FMD[,3],Mt$x)]*dim(FMD)[1]
  # estimation of the spatial intensity
  h&lt;-mse2d(as.points(FMD[,1:2]), Northcumbria, nsmse=50, range=4)
  h&lt;-h$h[which.min(h$mse)]
  Ms&lt;-kernel2d(as.points(FMD[,1:2]), Northcumbria, h, nx=5000, ny=5000)
  atx&lt;-findInterval(x=FMD[,1],vec=Ms$x)
  aty&lt;-findInterval(x=FMD[,2],vec=Ms$y)
  mhat&lt;-NULL
  for(i in 1:length(atx)) mhat&lt;-c(mhat,Ms$z[atx[i],aty[i]])
  # estimation of the STIK function
  u &lt;- seq(0,10,by=1)
  v &lt;- seq(0,15,by=1)
  stik1 &lt;- STIKhat(xyt=FMD, s.region=northcumbria/1000,t.region=c(1,200),
                   lambda=mhat*mut/dim(FMD)[1], dist=u, times=v, infectious=TRUE)
  ```
  
  ```{r}
  plotK(stik1)
  ```
  </code></pre>
  
  <p>after knitting, the plot doesnt appear in HTML. Does anyone has some idea what is going on?</p>
  
  <p>Thank you so much!</p>