Object not found error with ggplot2 when adding shape aesthetic

Question

I am attempting to add a shape aesthetic mapping to an existing plot but am receiving the error below. Is there a different way to accomplish this? If I remove shape=Port from the function call, everything works as expected.

p <- ggplot(data=w, aes(OAD,RtgValInt,color=dt,shape=Port)) +
    geom_jitter(size=3, alpha=0.75) +
     scale_colour_gradient(limits=c(min(w$dt), 
             max(w$dt)),
         low="#9999FF", high="#000066") +
     geom_point(data=data.frame(OAD=w$OAD[1], 
             RtgValInt=w$RtgValInt[1]), 
         color="red", size=3)
print(p)

Error in eval(expr, envir, enclos) : object 'Port' not found

The data frame w includes the data below.

Date          Port    OAD         RtgValInt   dt
12/31/2010  Grp1    1.463771    1.833333    14974
12/31/2010  Grp2    1.193307    2.071429    14974
11/30/2010  Grp1    1.454115    1.833333    14943
11/30/2010  Grp2    1.127755    2.071429    14943
10/29/2010  Grp1    1.434965    2.000000    14911
10/29/2010  Grp2    1.055758    2.071429    14911
09/30/2010  Grp1    1.441773    2.000000    14882
09/30/2010  Grp2    1.077799    2.071429    14882

Answer 1

Since every layer inherits the default aes mapping, you need to nullify the shape aes in geom_point when you use different dataset:

p <- ggplot(data=w, aes(OAD,RtgValInt,color=dt,shape=Port)) +
  geom_jitter(size=3, alpha=0.75) +
  scale_colour_gradient(limits=c(min(w$dt), 
      max(w$dt)),
    low="#9999FF", high="#000066") +
  geom_point(aes(shape=NULL), data=data.frame(OAD=w$OAD[1], 
      RtgValInt=w$RtgValInt[1]), 
    color="red", size=3)