R help - Sep 2008 - Setting user colors in barchart

Dear R Users,

I have a basis question regarding the use of color in the lattice package. I
read the ?barchart help page and searched the R archives but could not
understand how to do it.

I just need to plot a barchart using specific colors for my groups, e.g.
green and red instead of the default lattice colors. How do I do that?

If I say:
    barchart(x ~ a_factor, groups=my_groups, data=my_data,
col=c('green',
'red'), auto.key=TRUE)

then my barplot has the correct colors, but the legend does not. What is the
correct approach to set colors in barchart?

Thanks
Alex

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> I have a basis question regarding the use of color in the lattice 
package. I
> read the ?barchart help page and searched the R archives but could not
> understand how to do it.
> 
> I just need to plot a barchart using specific colors for my groups, e.g.
> green and red instead of the default lattice colors. How do I do that?
> 
> If I say:
>     barchart(x ~ a_factor, groups=my_groups, data=my_data, 
col=c('green',
> 'red'), auto.key=TRUE)
> 
> then my barplot has the correct colors, but the legend does not. What is 
the
> correct approach to set colors in barchart?
Two possibilities:
1. Manually specify the colours of the rectangles in the key by using the 
key argument instead of auto.key

my_cols <- c("green", "red", "blue")
barchart(yield ~ site, 
   groups=variety, 
   data = barley, 
   col=my_cols,
   key=list(text=list(levels(barley$variety)), 
rectangles=list(col=my_cols)))

2. Set superpose.polygon$col in the plot settings instead of using the col 
argument, e.g.

barchart(yield ~ site, 
   groups=variety, 
   data = barley, 
   auto.key=TRUE, 
   par.settings=list(superpose.polygon=list(col=my_cols)))

Regards,
Richie.

Mathematical Sciences Unit
HSL


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> Thank you for a clear example, it works. I tried to play with 
> superpose.polygon before to no avail, this clarifies things.
You're welcome.
> Another question: would you know how to add gridlines to the plot? 
> I'd like to have a few horizontal gridlines on my barchart plot for 
> better readability. Do I have to write a wrapper for panel.barchart?
Probably, but it's not as bad as it sounds.  To reuse the previous 
example:

barchart(yield ~ site,
  groups=variety,
  data = barley,
  panel=function(...){panel.barchart(...); panel.grid(v=0, ...)})

(The v=0 argument stop vertical grid lines being drawn.)

Regards,
Richie.

Mathematical Sciences Unit
HSL


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