[R] Histogram: plot by group

[Layo909]

I want to make a histogram in R of the data in attached excel file called
'cbt'. However, I need the histogram to show a separation for Group 1 and
Group 2, as in attached image. 

How do I do this in R? I know how to make a histogram for a single group,
but how can I separate the 2 groups?

http://r.789695.n4.nabble.com/file/n4256052/hist.jpg 

http://r.789695.n4.nabble.com/file/n4256052/cbt.xls cbt.xls 

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