Chapter 15: Plotly

In this module, you’ll start building visualizations using the Plotly API. Plotly is a visualization software that recently open-sourced it’s API to JavaScript, MatLab, Python, and R, making it quite valuable to learn. Plotly graphs are fairly customizable, and (by default) have a variety of interactive methods with each chart (i.e., hover, brush to zoom, pan, etc.). Many of these events are fairly cumbersome to build programmatically, which makes a library like Plotly quite attractive.

Helpful links:

• Plotly Website
• Plotly R API
• Getting Started with Plotly for R
• Plotly Cheatsheet
• Plotly book (extensive documentation)

15.1 Getting Started

The Plotly API is an R package that you’ll use to build interactive graphics. Like other open-source that we’ve used in this course, we’ll load this API as a R package as follows:

# Install package
install.packages("plotly")

# Load library
library(plotly)

Then, the plot_ly object will be accessible to you to build graphics on your page.

15.2 Basic Charts

One of the best ways to start building charts with Plotly is to take a look at a basic example of your choice, and explore the syntax. In general, to build a Plotly object (graph) you’ll pass a dataframe into the plot_ly function, then adjust the parameters to specify what you want to visualize. For example, here is the basic example of a scatterplot from the documentation:

# Make a scatterplot of the iris data
plot_ly(data = iris, x = ~Sepal.Length, y = ~Petal.Length, type = "scatter")

The approach seems pretty straightforward – in fact, if you exclude type = "scatter", Plotly will make an educated guess about what type of plot you want (and in this case, it will in fact create a scatterplot!). The only syntax that looks a bit strange is the tilde character (~). In R, the tilde designates a variable as a formula, which was a design choice of the developers of the API.

To practice making basic charts in Plotly, see exercise-1.

15.3 Layout

While the plot_ly function controls the data that is being visualized, additional chart options such as titles and axes are controlled by the layout function. The layout function accepts as a parameter a plotly object, and manipulates that object. Again, I think a great place to start is an example in the documentation:

# Create some data
Primates <- c('Potar monkey', 'Gorilla', 'Human', 'Rhesus monkey', 'Chimp')
Bodywt <- c(10.0, 207.0, 62.0, 6.8, 52.2)
Brainwt <- c(115, 406, 1320, 179, 440)
data <- data.frame(Primates, Bodywt, Brainwt)

# Create a plotly object
plot_ly(data, x = ~Bodywt, y = ~Brainwt, type = 'scatter',
        mode = 'text', text = ~Primates, textposition = 'middle right',
        textfont = list(color = '#000000', size = 16)) %>%
# Pass object to the layout function
        layout(title = 'Primates Brain and Body Weight',
             xaxis = list(title = 'Body Weight (kg)',
                          zeroline = TRUE,
                          range = c(0, 250)),
             yaxis = list(title = 'Brain Weight (g)',
                          range = c(0,1400)))

This example uses the pipe operator (%>%) to pass the plotly object into the layout function as the first argument. We can then infer the structure of the other parameters, which you can read about more in the API Documentation:

title: Accepts a string variable as the plot’s title.

xaxis: Accepts a named list to describe the rendering of the xaxis. See a full list of options here.

yaxis: Similarly to xaxis, Accepts a named list to describe the rendering of the yaxis. See a full list of options here.

These are of course not the only options you can specify, though are a good start. To practice using the layout function in Plotly, see exercise-2.