title: Getting an Error in ggplot: Invalid argument to unary operator
body: <p>I am working on a plot of some data values against months. </p>
  
  <p>I am using two <code>geom_lines()</code> function for each data type. I am getting the plot and everything I want, but when I try to label the x axis with "Months" I am getting an error. </p>
  
  <pre><code>Error in +xlab("Months") : invalid argument to unary operator
  </code></pre>
  
  <p>My code for the plot is : </p>
  
  <pre><code>library(ggplot2)
  library(officer)
  
  data &lt;- "U://30-Power &amp; Water//25 Renewables//WORK//Data//PVPlanner//PVPlanner.csv"
  
  data &lt;- read.table(data,skip = 36, header = T,  sep=";")
  
  months &lt;- c("Jan", "Feb", "Mar", "Apr", "May", "June", "July", "Aug", "Sep", "Oct", "Nov", "Dec")
  data &lt;- data.frame(months, data[1:12,2:6])
  data$months &lt;-factor(data$months,levels = c("Jan", "Feb", "Mar", "Apr", "May", "June", "July", "Aug", "Sep", "Oct", "Nov", "Dec"))
  
  plot &lt;- ggplot(data) +geom_path(aes(x= months, y= GHId, col = "GHId", group = 1)) + geom_path( aes(x = months,y = Diffd,colour = "Diffd", group = 1) )+ scale_colour_manual("", breaks =c("GHId","Diffd"), values = c( "red","Blue"))
  +xlab("Months")+ylabs("Irradiation")+ggtitle("Global and Diffuse horizontal irradiance")
  plot
  </code></pre>
  
  <p>And my data looks like this: </p>
  
  <pre><code>  GHId GHIm Diffd Diffm  T24
  1     3.27  101  0.92    29 13.3
  2     3.92  110  1.23    34 13.3
  3     5.30  164  1.58    49 13.9
  4     6.18  185  1.89    57 14.9
  5     6.93  215  2.00    62 16.4
  6     7.53  226  1.80    54 18.4
  7     7.42  230  1.87    58 21.1
  8     7.06  219  1.58    49 22.0
  9     5.97  179  1.39    42 21.3
  10    4.50  140  1.26    39 18.7
  11    3.53  106  0.99    30 15.9
  12    2.90   90  0.86    27 13.3
  </code></pre>
  
  <p>there is no issue at all with the plot. I am getting error only when I try to give labels to the axis. I am not sure why this happening.</p>
  
  <p>Your help will be appreciated. </p>
  
  <p>Regards</p>