Overlay decimal coordinates (New Jersey) on NAD83 Stateplane polygon in R - r

Question

I am trying to make a plot with points (decimal coordinates in New Jersey) on polyline shapefile with projection NAD 83 Stateplane (feet) (New Jersey). How can I do it? So far, I could plot the points and the shapefile separately but cannot overlay.
Plotted the shapefile using the following code:
orgListLayers("Counties.shp") # Shows the available layers for the shpaefile "Counties:
shape=readOGR("Counties.shp", layer="Counties") # Load the layer of the shapefile
plot(shape) # Plots the shapefile
Plotted points (vectors are lat1,long1) using the following code after transforming the points into Stateplane in ArcGIS:
dpts <- as.data.frame(cbind(long1,lat1))
plot(dpts2)
How can I overlay these points on the polyline shapefile?
Ultimately, I will have multiple sets of points which I want to plot on the shapefile as circles whose size would be dependent on values associated with the points. e.g. if each point represents a town, I want a bigger circle for a town having higher population.

Answer 1

You didn't provide any data, so this may be a partial answer.
Using the ggplot package it is easy to create layered maps. This map, of universities in NJ, was created with the code snippet that follows. It demonstrates plotting points and boundaries on the same map, and sizing the points based on a datum of the university (here, enrollment).
library(ggplot2)
library(rgdal)
setwd("<directory containing your data and maps")
states <- readOGR(dsn=".",layer="tl_2013_us_state")
nj.map <- states[states$NAME=="New Jersey",]
univ.map <- readOGR(dsn=".",layer="NJ_College_Univ_NAD83njsp")
nj.df <- fortify(nj.map)
univ.df <- univ.map#data
univ.df$ENROLL <- as.numeric(as.character(univ.df$ENROLL))
# create the layers
ggMap <- ggplot(nj.df)
ggMap <- ggMap + geom_path(aes(x=long,y=lat, group=group)) # NJ boundary
ggMap <- ggMap + geom_point(data=univ.df, aes(x=X, y=Y, size=ENROLL),color="red", alpha=0.7)
ggMap <- ggMap + coord_fixed()
ggMap <- ggMap + scale_size_continuous(breaks=c(5000,10000,15000,20000,25000,30000), range=c(0,10))
# render the map
ggMap
The TIGER/Line shapefile of US States was obtained here. The NJ Universities were obtained here.
Explanation:
The call to ggplot(...) defines the NJ map as the default dataset.
The call to geom_path(...) adds a layer to draw the NJ boundary.
The call to geom_point(...) adds a point layer locating the universities, with point size proportional to enrollment.
The call to coord_fixed(...) ensures that the map will not be distorted.
The call to scale_size_continuous(...) establishes breaks for the legend labels.