5.4 A ggplot bestiary
What follows is a quick run through of example
ggplots. These will predominantly be done by changing the geoms used, but there will be additional tweaks which we’ll highlight.
5.4.1 Density plot
Below is a density plot which is much like a histogram. The x axis shows observations of given numbers of flowers, while the y axis is the density of observations (roughly equivalent to number of rows with that many flowers, calculated in the background by the statistics layer). Each density is coloured according to nitrogen concentration, though note that we’re using
fill = instead of
colour =. Try using colour instead to see what happens.
Notice that we haven’t used
data = flower here and instead just used
flower? When an object is not assigned with an argument,
ggplot2 will assume that it is the dataset. We’re using that here, but we actually prefer to explicitly state the argument name in our own work.
Next is a histogram (a much more traditional version of a density plot). There are a couple of things to take note of here. The first is that
flower$block is numeric and not a factor. We can correct that here fairly easily using the
factor() function to convert it from numeric to factor (though ideally we’d have done this before - see Chapter 3). The other thing to take note of is that we’ve specified
bins = 20. The number of bins control how many times the y-axis is broken up to show the data. Try increasing and decreasing to see the effect. The last is using the
position = argument and stating that we do not want the bars to be stacked (the default position), instead we want them side-by-side, a.k.a. dodged.
5.4.3 Frequency polygons
A frequency polygon is yet another visualisation of the above two. The only difference here is that we are drawing a line to each value, instead of a density curve or bars.
Boxplots are a classic way to show to spread of data, and they’re easy to make in
ggplot2. The dark line in the middle of the box shows the median, the boxes show the 25th and 75th percentiles (which is different from the base R
boxplot()), and the whiskers show 1.5 times the inter-quartile range (i.e. the distance between between the first and third quartiles).
5.4.5 Violin plots
Violin plots are an increasingly popular alternative to boxplots. They display much of the same information, as well as showing a version of the density plot above (imagine each violin plot, cut in half vertically and place on it’s side, thus showing the overall distribution of the data). In the plot below the figure is slightly more complex than those above and so deserves some explanation.
geom_violin() we’ve included
draw_quantiles = where we’ve specified we want quantile lines drawn at the 25, 50 and 75 quantiles (using the
c() function). In combination with
geom_violin() we’ve also included
geom_jitter() is similar to
geom_point() but induces a slight random spread of the points, often helpful when those points would otherwise be clustered. Within
geom_jitter() we’ve also set
height = 0, and
width = 0.1 which specifies how much to jitter the points in a given dimension (here essentially telling
ggplot2 not to jitter by height, and only to jitter width by a small amount).
Finally, we’re also using this plot to show
scale_y_log10. Hopefully this is largely self-explanatory (it converts the y-axis to the log_10_ scale). There are additional scaling options for axis (for instance
scale_y_sqrt()). Please note that using a log scaled axis in this case is actually doing harm in terms of understanding the data, we’d actually be much better off not doing so in this particular case.
ggplot(flower) + geom_violin(aes(y = flowers, x = nitrogen, fill = nitrogen), draw_quantiles = c(0.25, 0.5, 0.75)) + geom_jitter(aes(y = flowers, x = nitrogen), colour = "black", height = 0, width = 0.1, alpha = 0.5) + scale_fill_manual(labels = c("High", "Medium", "Low"), values = c("#5f7f5c", "#749770", "#9eb69b")) + labs(y = "Number of Flowers", x = "Nitrogen Concentration") + scale_y_log10() + theme_rbook()
Below is an example of barcharts. It is included here for completeness, but be aware that they are viewed with contention (with good reason). Briefly, barcharts can hide information, or imply there is data where there is none; ultimately misleading the reader. There are almost always better alternatives to use that better demonstrate the data.
The barchart shows the numbers of observations in each block, with each bar split according to the number of observations in each nitrogen concentration. In this case they are equal because the dataset (and experimental design) was balanced.
5.4.7 Quantile lines
While we can draw a straight line, perhaps we would also like to include the descriptive nature of a boxplot, except using continuous data. We can use quantile lines in such cases. Note that for quantiles to be calculated
ggplot2 requires the installation of the package
Heatmaps are a great tool to visualise spatial patterns.
ggplot2 can easily handle such data using
geom_bin2d() to allow us to see if our data is more (or less) clustered.
In this example, lighter blue squares show combinations of leaf area and shoot area where we have more data, and dark blue shows the converse.
5.4.9 Hex map
A similar version to
geom_hex(). The only difference between the two is that the squares are replaced with hexagons. Note that
geom_hex() requires you to first install an additional package called
5.4.10 Contour map
Similar to a heatmap we can make a contour map using
geom_density_2d(). The way to read this figure is much the same way as you’d read a topographical map showing mountains or peaks. The central polygon represents the space (amongst shoot and leaf area) where there are most observations. As you “step” down this mountain to the next line, we step down in the number of counts. Think of this as showing where points are most clustered, as in
We can then expand on this using the statistics layer, via
stat_nameOfStatistic. For instance, we can use the calculated “level” (representing the height of contour) to fill in our figure. To do so, we’ll swap out
stat_density_2d() which will allow us to colour in the contour map.
In this case it doesn’t look too pretty given that we don’t have a wide spread of data leading to partly formed polygons.
5.4.11 Cleveland dotplot
The cleveland dotplot is a figure well suited to detect unusual oberservations (outliers?) in our data. The y axis is simply the order in which the data appears in the dataset. The x axis is our variable of interest. Points far to the left or far to the right are indicative of outliers.
ggplot2 does not have a geom specifically designed for a cleveland dotplot, though because of the grammar of graphics framework, we can build it ourselves.
We do this by first using the function
rownames(). This function returns the row name of each row. In most data frames, the row names are simply numbers. Once we have this information, we can convert this to numbers using
as.numeric(). After doing this we have a vector of numbers from 1 to 96. Keep in mind that such techniques can be used for any sort of figure we want to make. We’ll use the same technique to add horiztonal lines for each row of data to replicate the traditional look of a dotchart.
5.4.12 Pairs plot
For the final figure we will need an additional package, called
GGally is a function called
ggpairs() which is equivalent to
pairs() (part of base R). Both functions produce a scatterplot matrix of all variables of interest. Running diagonally down from left to right are plots showing distributions; either as stacked barcharts or as density plots, in this case coloured according to nitrogen concentration. The bottom plots show the plotted relationships, with variables on the top as the x axis, and the variables on the right plotted as the y axis. For example, the very bottom left plots shows the relationship between number of flowers on the y axis and treatment on the x axis, coloured according to nitrogen concentration. Conversely, the upper plots show either the correlation between variables, when both variables are continuous and the correlation can be calculated, boxplots when the plot is of a factor variable and a continuous variable, or as a stacked barchart when both variables are factors.
Such figures are a fantastic way to quickly show relationships in the dataset. They can of course be used to plot only part of the dataset, for instance using
flowers[ ,1:5] to plot the first five columns of the dataset.
We’ve included two additional arguments in the code below.
cardinality_threshold = forces
GGally to plot all of the variables in our dataset (where it normally wants fewer variables) as well as
progress = to avoid a progress bar appearing when we run the code.