R/ggplot2 adding an additional line/polygon to a choropleth map

Question

I have a choropleth map that represents the number of states being affected by a storm event. This is shown in the image below. I want to lay a transparent line or polygon over these states to represent the storm area. How do I do this?

Here is the code I'm using for ggplot:

ggplot() + geom_polygon(data = final.plot, aes(x = long, y = lat, group = group, fill = factor(T3)), color = "grey", size = 0.3) +
          coord_map("conic", lat0 = 30, xlim = c(-121, -72), ylim = c(25, 50)) + theme(axis.text = element_blank(), axis.ticks = element_blank(), axis.title = element_blank(), legend.position = "bottom") +
          geom_point(data=s1.lat.long, aes(x=long, y=lat), color="black", size=1)

Here is a link to the whole R code and shape files.

EDIT: I would like it to look like this mock image below, whereby a band spreads all the way across the US, representing geomagnetic storm activity. This band can then be made fairly transparent.

Really, this is a crude way of reproducing this image:

Answer 1

Edit this snippet into your code:

library(rgeos) # to use dissolve polygons
library(broom) # to replace fortify() with tidy().

# Select Affected states based on $T3. Subset and merge to one polygon.
statesAffectedList <- as.character(unique(final.plot$NAME_1[final.plot$T3 == "State Affected"]))
states.shp$STATE_NAME <- as.character(states.shp$STATE_NAME)
statesAffectedSHP <- gUnaryUnion(states.shp[states.shp$STATE_NAME %in% statesAffectedList, ])

stormArea <- tidy(statesAffectedSHP)  # tidy() replaces fortify(). See ?fortify for details

Later add stormArea to the plot using the following:

geom_polygon(data = stormArea, aes(x = long, y = lat, group = group), color = "black", alpha = 0, linetype = 1)

Note that you can change alpha parameter to add a fill effect to your polygon, and you can also control the border with the linetype. Following are the complete and adjusted plotting code and an image with the result.

ggplot() + geom_polygon(data = final.plot, aes(x = long, y = lat, group = group, fill = factor(T3)), color = "grey", size = 0.3) + coord_map("conic", lat0 = 30, xlim = c(-121, -72), ylim = c(25, 50)) +
theme(axis.text = element_blank(), axis.ticks = element_blank(), axis.title = element_blank(), legend.position = "bottom") +
geom_point(data=s1.lat.long, aes(x=long, y=lat), color="black", size=1) + 
geom_polygon(data = stormArea, aes(x = long, y =  lat, group = group), color = "black", alpha = 0, linetype = 1)