Interactive R Markdown Document with ggmap - r

Question

I'm working with the ggmap tutorial by Manuel Amunategui over at http://amunategui.github.io/ggmap-example/. It is a wonderful introduction to the ggmap package and thankfully I understand his tutorial.
However, I am also trying to make this material interactive through R markdown. When I run the below document, for some reason the rendering of the map is of very low quality. In my standard .R script the image produced is way better. Any thoughts as to what might cause the drastic difference in quality?
Also, in R Markdown, is it possible to have custom sizing of the images as well as placement? I am specifically interested in making the map larger and/or displaying another map with it side-by-side.
This first block of code is just to get your hands on the data if desired.
#install.packages("RCurl"); install.packages("xlsx"); install.packages("zipcode"); install.packages("ggmap")
library(RCurl)
library(xlsx)
# NOTE if you can't download the file automatically, download it manually at:
#'http://www.psc.isr.umich.edu/dis/census/Features/tract2zip/'
urlfile <-'http://www.psc.isr.umich.edu/dis/census/Features/tract2zip/MedianZIP-3.xlsx'
destfile <- "census20062010.xlsx"
download.file(urlfile, destfile, mode="wb")
census <- read.xlsx2(destfile, sheetName = "Median")
#census <- read.xlsx2(file = "census20062010.xlsx", sheetName = "Median")
head(census)
# clean up data
# census <- census[c('Zip','Median..', 'Pop')]
names(census) <- c('Zip','Median', 'Pop')
census$Median <- as.character(census$Median)
census$Median <- as.numeric(gsub(',','',census$Median))
census$Pop <- as.numeric(gsub(',','',census$Pop))
head(census)
# get geographical coordinates from zipcode
library(zipcode)
data(zipcode)
census$Zip <- clean.zipcodes(census$Zip)
census <- merge(census, zipcode, by.x='Zip', by.y='zip')
census$location <- paste0(census$city, ", ", census$state)
names(census) <- sapply(names(census), tolower)
# saved census to census.rdata at this point...
The next chunk of code below is what is in the markdown file.
```{r, message=FALSE, echo=FALSE}
library(ggmap)
library(ggplot2)
load("census.rdata")
inputPanel(
textInput("loc", label = "Location", value = "Orlando, FL"),
sliderInput("zoom", label = "Zoom Level",
min = 1, max = 12, value = 10, step = 1)
)
renderPlot({
census2 <- census[census$location == input$loc,]
map <- get_map(location = input$loc,
zoom = input$zoom,
maptype = 'roadmap',
source = 'google',
color = 'color',
filename = "ggmapTemp")
print(ggmap(map) +
geom_point(
aes(x=longitude, y=latitude,
show_guide = TRUE, size=Median),
data=census2, colour = I('red'), na.rm = T)
)
})
```
Thanks for your help!