Density Plots and Histograms in ggplot2

In this post, I'm going to go through how to make plots of distributions (either density plots or histograms) in ggplot2. I'm going to draw upon examples of Fisherian testing in the context of causal inference, but the examples should be completely understandable without knowledge of Fisher's approach to inference.

For example, suppose that we want to plot the randomization distribution of a test statistic (difference in means in this case) under the sharp null of zero treatment effect across all units. The possible values of the test statistic across 1000 randomly sampled randomizations are in the vector test.stat. Moreover, the actually observed test statistic is tau.hat. Not only do we want to plot the density, but we want to shade in the area under the density that corresponds to the one-sided p-value of this test statistic.

We can first turn the vector into a data frame that contains the values as x and the corresponding density as y:

dd <- with(density(test.stat),data.frame(x,y))

Then plot in ggplot:

ggplot(data = dd, mapping = aes(x = x, y = y)) +
geom_line(color="black") + layer(data = dd, mapping = aes(x=ifelse(x < tau.hat,x,tau.hat), y=y), geom = "area", geom_params=list(fill="red",alpha=.3)) + 
scale_y_continuous(limits = c(0,max(dd$y)), name="Density") + 
scale_x_continuous(name="Difference in Means") + 
geom_vline(aes(xintercept=tau.hat.1a), color="red", linetype="dashed") + 
geom_text(mapping=aes(x2,y2,label = text2), data=data.frame(x2=-42, y2=0.015, text2="T[obs]"), color=I("red"), parse=TRUE) + 
geom_text(mapping=aes(x2,y2,label = text2), data=data.frame(x2=-60, y2=0.0025, text2="p-value = 0.041"), size=3)

Several comments on the syntax passed to the ggplot() function above:

• The layer() function with the geom="area" option fills the area under the distribution corresponding to the one-sided p-value (probability of observing a value of the test statistic at least as small as the one we observe).
• I manually pass the location of the text into the geom_text() function in this case by creating a data frame within the actual geom_text() function.
• Note that in order for ggplot to write expressions with text, such as subscripts which I use above, I pass the parse=TRUE option to geom_text()

Here's another example that involves plotting the distribution of p-values under the sharp null of a constant additive treatment effect for each unit. I wanted to plot the values of the resultant test statistics on the x-axis, the p-values for the sharp null on the y-axis, and then shade the 95% Fisherian confidence interval. In this case, tau.vector contains the values of the test statistic (difference in means). p.vector is a corresponding vector that contains the one-sided exact Fisher p-value for the sharp null hypothesis that the effect is 0. Note that I placed a dotted horizontal red line at a p-value of 0.025 to obtain approximately 95% coverage on my confidence interval.

ggplot(data = CI.2c, mapping = aes(x = tau.vector, y = p.vector)) +
geom_line(color="black") + 
scale_y_continuous(limits = c(0,max(CI.2c$p.vector)),name="p-value") +
scale_x_continuous(name="Difference in Means") + 
layer(data = CI.2c, mapping = aes(x=ifelse((lb.2c < tau.vector & tau.vector < ub.2c),tau.vector,lb.2c), y=p.vector), geom = "area", geom_params=list(fill="yellow",alpha=.3)) + 
geom_hline(aes(yintercept=0.025), color="red", linetype="dashed") + 
geom_text(mapping=aes(x2,y2,label = text2), data=data.frame(x2=-100, y2=0.1, text2="95% CI: [-83,6.33]"), size=5)

Here's a very simple histogram example. Suppose that pval.5 contains a vector of p-values across all possible assignment vectors. We want to plot the distribution of Fisher's exact p-values, which should be uniformly distributed between 0 and 1 under the null (and generally when dealing with continuous test statistics). We can do so simply using ggplot as:

ggplot() + 
geom_histogram(aes(x = pval.5), binwidth=0.1, linetype=1, alpha=.25, color="black", fill="red") +
scale_y_continuous(name="Count") + 
scale_x_continuous(name="p-value")