[R] Plot different regression models on one graph

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Rhonda Reidy wrote:
> 
> The following variables have the following significant relationships (x is
> the explanatory variable): linear, cubic, exponential, logistic. The
> linear relationship plots without any trouble. 
> 
> Cubic is the 'best' model, but it is not plotting as a smooth curve using
> the following code:
> 
> cubic.lm<- lm(y~poly(x,3))
> lines(x,predict(cubic.lm),lwd=2)
> 
> How do I plot the data and the estimated curves for all of these
> regression models in the same plot? 
> 
> x <- c(62.5,68.5,0,52,0,52,0,52,23.5,86,0,0,0,0,0,0,0,0,0,0)
> 
> y <-
> c(0.054,0.055,0.017,0.021,0.020,0.028,0.032,0.073,0.076,0.087,0.042,0.042,0.041,0.045,0.021,0.018,0.017,0.018,0.028,0.022)
> 

Hi Rhonda,

The problem is that the default behavior of predict() only produces output
corresponding to the original values of x used in the model.  To get a
"smooth" curve, you will want to predict at a large number of x values that
fall within the range of the data.  This can be done using the pretty()
function, or using a composition of the seq() and range() functions if you
desire more control:

  # Create a new vector of X values at which to generate predictions:
  predX <- pretty( x, n = 100 )

  plot( y ~ x )

  # Add the regression model, but generate predictions using the
  # points in predX-- the default is to predict using points in x.

  lines( predX, predict( cubic.lm, newdata = list( x = predX )), lwd = 2 )


The above should produce the output you want.  However, I suggest Hadley
Wickham's excellent ggplot2 package-- I feel the following is more
expressive than using base graphics functions:

  require( ggplot2 )
  Plot <- qplot( x, y ) + stat_smooth( method = 'lm', formula = 'y ~
poly(x,3)' )
  print( Plot )

The amazing power of ggplot2 is that when you want to alter or extend your
plot, you simply "add" more ggplot2 commands to your plot object:

  # Add a linear regression model and change the color theme used in the
output
  Plot <- Plot + stat_smooth( method = 'lm', color = 'red' ) + theme_bw()
  print( Plot )


Hope this helps!

-Charlie
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