title: What is the reason of the error: non-numeric argument to binary operator in R
body: <p>I am trying to make grouped (by year) bar-plot with separate columns based on anomaly data (+/-). 
  I have used following script and data in R.</p>
  
  <pre><code>mydata &lt;- read.csv("F:/MOD13A1_NDVI_500/Mod_ndvi_500_excel/ndvi_anomaly.csv", head=TRUE)
  mydata
  
          NZ       X2000  X2001  X2002  X2003  X2004  X2005  X2006  X2007  X2008
  1 High_mountain  0.007 -0.003 -0.002 -0.016  0.011  0.016 -0.007  0.000 -0.003
  2         Taiga -0.002  0.018 -0.006 -0.022  0.018  0.004 -0.016  0.025  0.003
  3 Forest_steppe  0.004  0.011 -0.044 -0.008  0.009  0.003 -0.004 -0.005 -0.001
  4        Steppe  0.001 -0.016 -0.002  0.007 -0.022 -0.004 -0.017 -0.053  0.000
  
  par(xpd=T, mar=par()$mar+c(0,0,0,6))
  barplot(as.matrix(mydata[1:6,]), beside=T)
  </code></pre>
  
  <p>It returns error: </p>
  
  <pre><code>Error in -0.01 * height : non-numeric argument to binary operator
  </code></pre>
  
  <p>What is the reason of this kind of error? I have found several questions with error non-numeric argument to binary operator in this site, but each cases were different. I thought it's cause might be negative(-) values. How to avoid from this error?</p>