Show or Hide Grid

### ** Examples

#Load data
  data(Feldspar)
  plot <- ggtern(data=Feldspar,aes(Ab,An,Or)) +
          geom_point()   + #Layer
          theme_bw()     + #For clarity
          theme_hidegrid() #Turn off both major and minor
  plot

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#Demonstrate switching on major, minor and both gridlines
  plot + theme_showgrid_minor() #show minor only

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plot + theme_showgrid_major() #show major only

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plot + theme_showgrid()       #show both major and minor

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#Demonstrate switching OFF major, minor and both gridlines, Uncomment to run
  #plot <- plot + theme_showgrid() #as before
  #plot + theme_hidegrid_minor() #show major only (hide minor)
  #plot + theme_hidegrid_major() #show minor only (hide major)
  #plot + theme_hidegrid()       #show none (hide both major and minor)