First Steps

Introduction

The goal of this chapter is to show you how to produce useful graphics with gongbi.

Fuel economy data

In this chapter, we will use the mpg dataset from the ggplot2 package. The dataset includes information about the fuel economy of popular car models in 1999 and 2008, collected by the US Environmental Protection Agency.

// Read the dataset to a polars DataFrame
let mpg = ...
println!("{mpg:?}");

shape: (234, 11)
┌──────────────┬────────┬───────┬──────┬───┬─────┬─────┬─────┬─────────┐
│ manufacturer ┆ model  ┆ displ ┆ year ┆ … ┆ cty ┆ hwy ┆ fl  ┆ class   │
│ ---          ┆ ---    ┆ ---   ┆ ---  ┆   ┆ --- ┆ --- ┆ --- ┆ ---     │
│ str          ┆ str    ┆ f64   ┆ i64  ┆   ┆ i64 ┆ i64 ┆ str ┆ str     │
╞══════════════╪════════╪═══════╪══════╪═══╪═════╪═════╪═════╪═════════╡
│ audi         ┆ a4     ┆ 1.8   ┆ 1999 ┆ … ┆ 18  ┆ 29  ┆ p   ┆ compact │
│ audi         ┆ a4     ┆ 1.8   ┆ 1999 ┆ … ┆ 21  ┆ 29  ┆ p   ┆ compact │
│ audi         ┆ a4     ┆ 2.0   ┆ 2008 ┆ … ┆ 20  ┆ 31  ┆ p   ┆ compact │
│ audi         ┆ a4     ┆ 2.0   ┆ 2008 ┆ … ┆ 21  ┆ 30  ┆ p   ┆ compact │
│ audi         ┆ a4     ┆ 2.8   ┆ 1999 ┆ … ┆ 16  ┆ 26  ┆ p   ┆ compact │
│ …            ┆ …      ┆ …     ┆ …    ┆ … ┆ …   ┆ …   ┆ …   ┆ …       │
│ volkswagen   ┆ passat ┆ 2.0   ┆ 2008 ┆ … ┆ 19  ┆ 28  ┆ p   ┆ midsize │
│ volkswagen   ┆ passat ┆ 2.0   ┆ 2008 ┆ … ┆ 21  ┆ 29  ┆ p   ┆ midsize │
│ volkswagen   ┆ passat ┆ 2.8   ┆ 1999 ┆ … ┆ 16  ┆ 26  ┆ p   ┆ midsize │
│ volkswagen   ┆ passat ┆ 2.8   ┆ 1999 ┆ … ┆ 18  ┆ 26  ┆ p   ┆ midsize │
│ volkswagen   ┆ passat ┆ 3.6   ┆ 2008 ┆ … ┆ 17  ┆ 26  ┆ p   ┆ midsize │
└──────────────┴────────┴───────┴──────┴───┴─────┴─────┴─────┴─────────┘

Key components

Every gongbi plot is composed of three main components:

1. Data that you want to visualize,
2. A set of aesthetic mappings that describe how variables in the data are mapped to visual properties, and
3. At least one layer that describes how to display the data. Layers are usually created with a geom_* macro.

Here is a simple example:

use gongbi::{aes, geom_point, labs, plot};
use polars::prelude::*;

fn main() -> anyhow::Result<()> {
    let mpg = CsvReadOptions::default()
        .with_has_header(true)
        .try_into_reader_with_file_path(Some("mpg.csv".into()))?
        .finish()?;

    let plot = plot!(mpg.clone(), aes!("displ", "hwy"), save = "mpg_point.svg")
        + geom_point!()
        + labs!(caption = "Demo of geom_point");

    plot.draw()?;

    let plot = plot!(
        mpg.clone(),
        aes!("displ", "hwy", color = "blue"),
        save = "mpg_point_color.svg"
    ) + geom_point!();
    plot.draw()?;

    Ok(())
}

This produces a scatterplot defined by:

1. Data: mpg dataframe,
2. Aesthetic mappings: x = "displ", y = "hwy",
3. Layer: geom_point().

In the above example, data and aesthetic mappings are supplied in plot!(), then layers are added on with + operator.

Almost every plot needs data and aesthetic mappings, and most aesthetic mappings are about which columns in the data are mapped to x and y axes. Thus, plot!() and aes!() macros accept unnamed arguments to simplify the usage. That is, the following lines are equivalent:

plot!(data = mpg, mapping = aes!(x = "displ", y = "hwy"))
plot!(mpg, mapping = aes!(x = "displ", y = "hwy"))
plot!(mpg, aes!(x = "displ", y = "hwy"))
plot!(mpg, aes!("displ", y = "hwy"))

For simplicity, we will use the last form in this book.

Color, size, shape and other aesthetic attributes

In addition to x and y axes, you can also provide other aesthetic mappings to control the appearance of the data points.

Color

use gongbi::{aes, geom_point, labs, plot};
use polars::prelude::*;

fn main() -> anyhow::Result<()> {
    let mpg = CsvReadOptions::default()
        .with_has_header(true)
        .try_into_reader_with_file_path(Some("mpg.csv".into()))?
        .finish()?;

    let plot = plot!(mpg.clone(), aes!("displ", "hwy"), save = "mpg_point.svg")
        + geom_point!()
        + labs!(caption = "Demo of geom_point");

    plot.draw()?;

    let plot = plot!(
        mpg.clone(),
        aes!("displ", "hwy", color = "blue"),
        save = "mpg_point_color.svg"
    ) + geom_point!();
    plot.draw()?;

    Ok(())
}