Maps are great – German Gas Prices illustrated

One of the most appealing data visualisation charts are maps.
I love maps as they combine an incredible information density with intuitive readability.
Also I feel that most people prefer maps over other visualisations. (Is there research on this?)
So it is time to get R-map-ready.

As a play example, I downloaded all German gas stations which are next to the “Autobahn”. Along with the names, I got the exact locations (in form of latitude/longitude) and the price of gasoline at each station. (Prices are in Euro and taken on a Friday night in a time span of roughly 30 minutes.)
For starters, we just plot all gas stations on a map and color them depending on their price for (super e5) gasoline.

The Autobahn is clearly marked by the yellow/red dots.

In a second step let’s create a density-based map. It ignores prices but takes the 2D-density of gas stations into consideration. It answers the question: where are the most gas stations per square mile?

A more interesting question might be: are there regional clusters with higher prices?
In order to illustrate regional prices, we can cluster prices regionally (stat_summary_2d) and plot them as tiles on top of the map.

While that gives some insight, it feels clunky.
A better way is to cluster the stations by price (using cut2) first, then show the cluster density on individual maps (with facet_wrap).

I hope that short play example showed what can be (easily) done with maps in R/ggplot.

Here are the 3 take-aways:
1. use `stat_density_2d(geom = "polygon", bins = 30,data=dfff, aes(x = lon, y = lat, alpha=..level.., fill = ..level..))` to plot the DENSITY of x/y coordinates on a map.
2. use `stat_summary_2d(geom = "tile",bins = 50,data=dfff, aes(x = lon, y = lat, z = price)` to plot the AGGREGATION of a third variable (e.g. Price) on a map
3. `options(stringsAsFactors=T)` needs to be set, in order for `stat_density_2d(geom = "polygon" )` to work; for more “details”, see Stackoverflow.

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