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: