Step 6 of 8

Bar charts for categories

A line chart has numbers on both axes, and its line says that the values change from one x to the next. Often the x values are names instead: days of the week, products, kinds of books. Such values are categories, and a bar chart compares them, with a bar per category whose height is its value.

A bar chart

A city's bike share counts the trips started on each day of one week:

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.chart.BarChart;
import javafx.scene.chart.CategoryAxis;
import javafx.scene.chart.NumberAxis;
import javafx.scene.chart.XYChart;
import javafx.stage.Stage;

public class Main extends Application {

    @Override
    public void start(Stage stage) {
        CategoryAxis days = new CategoryAxis();
        days.setLabel("Day");
        NumberAxis trips = new NumberAxis();
        trips.setLabel("Trips");
        BarChart<String, Number> chart = new BarChart<>(days, trips);
        chart.setTitle("Bike share trips in a week");
        chart.setLegendVisible(false);

        XYChart.Series<String, Number> week = new XYChart.Series<>();
        week.setName("Trips");
        week.getData().add(new XYChart.Data<>("Mon", 340));
        week.getData().add(new XYChart.Data<>("Tue", 310));
        week.getData().add(new XYChart.Data<>("Wed", 365));
        week.getData().add(new XYChart.Data<>("Thu", 330));
        week.getData().add(new XYChart.Data<>("Fri", 405));
        week.getData().add(new XYChart.Data<>("Sat", 520));
        week.getData().add(new XYChart.Data<>("Sun", 470));
        chart.getData().add(week);

        stage.setScene(new Scene(chart));
        stage.setTitle("Bike share");
        stage.show();
    }

    public static void main(String[] args) {
        launch(Main.class);
    }
}

The window

This window is wider than the screen: scroll it sideways, or read it as text below.

The window as text

Window "Bike share"
  BarChart "Bike share trips in a week" (no legend)
    x axis CategoryAxis "Day"
    y axis NumberAxis "Trips"
    series "Trips": ("Mon", 340) ("Tue", 310) ("Wed", 365) ("Thu", 330) ("Fri", 405) ("Sat", 520) ("Sun", 470)

A CategoryAxis is an axis of text values. Each category gets a place of its own along the axis, in the order the categories first appear in the data: here Mon to Sun, as they were added.
BarChart<String, Number>: the x values are Strings and the y values Numbers, so the series and the points have the same two types, XYChart.Series<String, Number> and new XYChart.Data<>("Mon", 340).
Everything else is as in a line chart: titles, labels, series, legend. In the window as text, a category is in quotes: ("Mon", 340).
A bar chart needs one category axis and one number axis. With the number axis first, BarChart<Number, String>, the bars lie sideways, which suits long category names.

Totals from a file

The bike share's file has a row per station and day. A station that was closed for the day (for repairs) has a -:

The file trips.csv

day,station,trips
Mon,Harbor,180
Mon,Market,160
Tue,Harbor,150
Tue,Market,-
Wed,Harbor,170
Wed,Market,195
Thu,Harbor,165
Thu,Market,165

The chart of the days needs each day's total, so the program adds the rows up in a map from the day to its total, as module 31 counted things. It also needs the days in order, and a HashMap's keys don't keep one, so a list keeps the days in the order they first appear in the file:

import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.chart.BarChart;
import javafx.scene.chart.CategoryAxis;
import javafx.scene.chart.NumberAxis;
import javafx.scene.chart.XYChart;
import javafx.stage.Stage;

public class Main extends Application {

    @Override
    public void start(Stage stage) throws Exception {
        Map<String, Integer> totals = new HashMap<>();
        List<String> days = new ArrayList<>();
        List<String> lines = Files.readAllLines(Paths.get("trips.csv"));
        for (int i = 1; i < lines.size(); i++) {
            String[] parts = lines.get(i).split(",");
            String day = parts[0];
            if (!days.contains(day)) {
                days.add(day);
                totals.put(day, 0);
            }
            if (!parts[2].equals("-")) {
                totals.put(day, totals.get(day) + Integer.valueOf(parts[2]));
            }
        }

        CategoryAxis dayAxis = new CategoryAxis();
        dayAxis.setLabel("Day");
        NumberAxis tripAxis = new NumberAxis();
        tripAxis.setLabel("Trips");
        BarChart<String, Number> chart = new BarChart<>(dayAxis, tripAxis);
        chart.setTitle("Trips per day, all stations");
        chart.setLegendVisible(false);
        XYChart.Series<String, Number> series = new XYChart.Series<>();
        series.setName("Trips");
        for (String day : days) {
            series.getData().add(new XYChart.Data<>(day, totals.get(day)));
        }
        chart.getData().add(series);

        stage.setScene(new Scene(chart));
        stage.setTitle("Bike share");
        stage.show();
    }

    public static void main(String[] args) {
        launch(Main.class);
    }
}

The window

This window is wider than the screen: scroll it sideways, or read it as text below.

The window as text

Window "Bike share"
  BarChart "Trips per day, all stations" (no legend)
    x axis CategoryAxis "Day"
    y axis NumberAxis "Trips"
    series "Trips": ("Mon", 340) ("Tue", 150) ("Wed", 365) ("Thu", 330)

A day enters the list and the map (with the total 0) at its first row, and every row with a number adds to its day's total. A closed station adds nothing to the total: no trips started there.
The bars come from the list, in its order, and each bar's value from the map.

Bars side by side

To compare the stations, each station gets a series of its own, as each plant did in step 3. A bar chart puts the bars of a category side by side, one per series, and the legend tells the series apart:

The file trips.csv

day,station,trips
Mon,Harbor,180
Mon,Market,160
Tue,Harbor,150
Tue,Market,-
Wed,Harbor,170
Wed,Market,195
Thu,Harbor,165
Thu,Market,165

import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.chart.BarChart;
import javafx.scene.chart.CategoryAxis;
import javafx.scene.chart.NumberAxis;
import javafx.scene.chart.XYChart;
import javafx.stage.Stage;

public class Main extends Application {

    @Override
    public void start(Stage stage) throws Exception {
        Map<String, XYChart.Series<String, Number>> byStation = new HashMap<>();
        List<String> lines = Files.readAllLines(Paths.get("trips.csv"));
        for (int i = 1; i < lines.size(); i++) {
            String[] parts = lines.get(i).split(",");
            String station = parts[1];
            if (!byStation.containsKey(station)) {
                XYChart.Series<String, Number> series = new XYChart.Series<>();
                series.setName(station);
                byStation.put(station, series);
            }
            int trips = 0;
            if (!parts[2].equals("-")) {
                trips = Integer.valueOf(parts[2]);
            }
            byStation.get(station).getData().add(new XYChart.Data<>(parts[0], trips));
        }

        CategoryAxis dayAxis = new CategoryAxis();
        dayAxis.setLabel("Day");
        NumberAxis tripAxis = new NumberAxis();
        tripAxis.setLabel("Trips");
        BarChart<String, Number> chart = new BarChart<>(dayAxis, tripAxis);
        chart.setTitle("Trips per station");
        List<String> stations = new ArrayList<>(byStation.keySet());
        Collections.sort(stations);
        for (String station : stations) {
            chart.getData().add(byStation.get(station));
        }

        stage.setScene(new Scene(chart));
        stage.setTitle("Bike share");
        stage.show();
    }

    public static void main(String[] args) {
        launch(Main.class);
    }
}

The window

This window is wider than the screen: scroll it sideways, or read it as text below.

The window as text

Window "Bike share"
  BarChart "Trips per station"
    x axis CategoryAxis "Day"
    y axis NumberAxis "Trips"
    series "Harbor": ("Mon", 180) ("Tue", 150) ("Wed", 170) ("Thu", 165)
    series "Market": ("Mon", 160) ("Tue", 0) ("Wed", 195) ("Thu", 165)

Here a closed station gets a bar of 0 for its day, on purpose: every series then has the same categories in the same order, and the empty place next to Harbor's Tuesday bar shows that Market had no trips. (A - that meant "not measured" would instead be left out, as in step 1: then no bar, rather than a bar of 0.)
The stations are sorted by name, as the plants were, so the series are in a fixed order.

Your task

Challenge 1 of 3

The school library counts its loans per genre in genres.csv:

genre,loans
Fantasy,128
Mystery,96
Comics,152
Science,61

Fill in the blanks of a bar chart with a category axis labeled Genre, an auto-ranging number axis labeled Loans, the title Loans by genre, no legend, and one series, Loans, with a bar for each row, in the order of the file.

Challenge 2

A stand at a farmers' market writes down each sale as a row, apples,3: the product and the kilograms (whole numbers). For a bar chart of the day, it needs the totals and the products in order. Write the class SalesLog in SalesLog.java, with two methods:

public static Map<String, Integer> totals(List<String> rows): a map from each product to the sum of its kilograms
public static List<String> products(List<String> rows): each product once, in the order it first appears in the rows

The rows have no header. No JavaFX in it. The check runs:

List<String> rows = new ArrayList<>();
rows.add("apples,3");
rows.add("pears,2");
rows.add("apples,5");
rows.add("plums,1");
rows.add("pears,4");
System.out.println(SalesLog.products(rows));
Map<String, Integer> totals = SalesLog.totals(rows);
System.out.println(totals.get("apples") + " " + totals.get("pears") + " " + totals.get("plums"));

and expects:

[apples, pears, plums]
8 6 1

Challenge 3

A small cinema shows three films, each in an afternoon and an evening show, and writes the tickets sold in tickets.csv, one column per show. A show that was canceled has a -:

film,Afternoon,Evening
Cartoon,64,22
Nature film,31,48
Comedy,40,-

Write start so that the window, titled Cinema, shows a bar chart (the scene's root) with the title Tickets sold, a category axis labeled Film, an auto-ranging number axis labeled Tickets and a visible legend. It has a series for each show, named as in the header and in its order, with a bar (film, tickets) for each film, in the order of the file; a canceled show has no bar for that film. The file may have any number of shows. The task card shows the window for the file above.

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