[R] Constrain density to 0 at 0?

[Farley, Robert]

"Read the help page for density more carefully. Especially the bw and  
from arguments."


Yes,  it was my inability to make sense of the help page that motivated my email.

My distances range from 0.4 to 7.6 but the density plot ranges from -2 to 10.  

"from=0" seems to hide the negative portion of the density without setting it to 0.


I've tried adjust, but it requires a value of 0.2, which results in a very, very spiky plot.


I was hoping for a process that could "forbid" impossible (negative) values, yet allow the density plot to blend the individual measurements. Is there a "variable bw" that could be set small at the extrema, and larger in the range of the data?




 
Robert Farley
Metro
www.Metro.net 
 
-----Original Message-----
From: David Winsemius [mailto:dwinsemius at comcast.net] 
Sent: Monday, July 19, 2010 19:31
To: Farley, Robert
Cc: r-help at r-project.org
Subject: Re: [R] Constrain density to 0 at 0?


On Jul 19, 2010, at 9:57 PM, Farley, Robert wrote:

> I'm plotting some trip length frequencies using the following code:
>
> plot( density(zTestData$Distance, weights=zTestData$Actual),
>    xlim=c(0,10),
>    main="Test TLFD",
>    xlab="Distance",
>   col=6  )
> lines(density(zTestData$Distance, weights=zTestData$FlatWeight),  
> col=2)
> lines(density(zTestData$Distance, weights=zTestData$BrdWeight ),  
> col=3)
>
> which works fine except the distances are all positive, but the  
> densities don't drop to 0 until around -2 or -3.
>
> Is there a way for me to "force" the density plot to 0 at 0?

Yes. (Assuming it can be zero, given the data.)

Read the help page for density more carefully. Especially the bw and  
from arguments.

-- 
David.