TAChart Tutorial: Userdefined ChartSource

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Introduction

In this tutorial we use TAChart to plot the age distribution of world population. In doing this, you will learn

• how to work with an area series
• and, most of all, how to apply a user-defined chart source

As usual, we assume that you are familiar with Lazarus and the Object Pascal language and have some basic knowledge of the TAChart package that you would acquire by working through the Getting Started tutorial.

Preparation

Data

The site www.census.gov contains a wealth of demographic data. This link leads you directly to the data that we want to plot: world population as a function of age and gender.

Go to that page, select a year and click "submit". In the lower part of the window, you will see a table with the columns "Age", "Both Sexes Population", "Male population", "Female population", and "Sex ratio". Use the mouse to select the data in the table (including header) and copy them to the clipboard. Paste the data into a text editor and save as "population.txt".

Open Lazarus and create a new project. At first we have to load the data to make them available for plotting.

We define a TPopulationRecord and an array of type TPopulationArray to hold the data of the population table:

type
  TPopulationRecord = record
    Age: Integer;
    Total: Double;
    Male: Double;
    Female: Double;
    Ratio: Double;
  end;

  TPopulationArray = array of TPopulationRecord;

Now let's read the file. Have a look at the unit population.pas in the Source Code section of this tutorial. You will find there a

procedure LoadPopulationData(const AFilename:string; var Data:TPopulationArray)

which does the file reading and stores the data in a TPopulationArray.

Just include the unit population in your uses list to get access to this procedure. Add a variable PopulationData of type TPopulationArray to the private section of the main form. Now we have everything ready to call LoadPopulationData from the form's OnCreate event handler. This will read our data file into the array PopulationData.

uses
  ..., population;

type
  TForm1 = class(TForm)
  // ...
  private
    PopulationData : TPopulationArray;
  // ...
  end;

const
  POPULATION_FILE = 'population.txt';

procedure TForm1.FormCreate(Sender: TObject);
begin
  LoadPopulationData(POPULATION_FILE, PopulationData); 
end;

Compile to check for obvious errors. You may get a runtime error on that the data file cannot be found. This is because our data file is assumed to reside in the same directory as the exe file. To fix this, either change the constant POPULATION_FILE to the correct storage location, or copy "population.txt" to the exe folder. Now the program should run fine, you will see an unspectacularly empty form, though.

Combobox as category selector

The file contains various categories: total, male, and femal population, as well as male-to-female ratio. Why not display these data in our chart?

To prepare for this,

• add a panel to the form, align it to alTop, and delete its caption.
• Add a combobox to this panel, set its Style to csDropdownList, add the strings "Total population", "Male population", "Female population", and "Ratio male/female (%)" to the Items property, and set ItemIndex to 0.

Preparing TChart

Now we are finished with preparations, and it's time for charting...

• Add a TChart component to the form, align it to alClient.
• Set the chart's BackColor to clWhite and the Grid.Color of each axis to clSilver.
• Get axis titles shown by setting LeftAxis.Visible and BottomAxis.Visible to true.
• Set the caption of the bottom axis to "Age", and that of the left axis to "Total population" (or leave it empty - we will later replace it at runtime by the selected item of the combobox).
• Use the text "World population" as the chart's title.
• Set these font styles to fsBold.
• Maybe it is a good idea to display our data reference in the footer - the chart property Foot can be used for that. Note that the property editor of Foot.Text (as well as that of Title.Text) allows to enter linefeeds for multi-lined titles.

Creating an area series

This time, we want to display our data as an area series. This type of series connects the data points as in a line series, but also fills the area underneath by a given color. It is very common to display demographic data, like a population pyramid, in this way.

So, add a TAreaSeries to the chart. As usual, you don't see it at this time because it does not yet have any data. But we can prepare some properties:

• Set its SeriesColor to clSkyBlue. This is the fill color of the area series.
• Hide the vertical connecting lines by setting the AreaLinesPen to psClear.
• If you want you can also change ConnectType to ctStepXY or ctStepYX to get a step effect.

Later, when the series has data, you should play around with these and other properties to see what they do.

User-defined chart source

Now, how do we get our data to the area series?

The first idea might be iterate through our PopulationData array and copy the data to the series by means of AddXY calls, like this:

var
  i: Integer;
begin
  for i:=0 to High(PopulationData) do
    Chart1AreaSeries1.AddXY(PopulationData[i].Age, PopulationData[i].Total);
end;

The disadvantage of this approach is that we would have the data in memory twice: in the PopulationData array and in the series - not so good: we would be wasting memory, and we would have to synchronize both storages when data points are added, deleted or edited.

To avoid this, we add a TUserDefinedChartSource to the form, it is the third or fourth icon from the left in the Chart component palette. This chart source is made to interface to any kind of data storage, the source itself does not store any data. You just have to write an event handler for OnGetChartDataItem in order to transfer the data. For this purpose, the event takes a var parameter AItem of type TChartDataItem that is defined as follows (with elements omitted that are not needed here):

type
  TChartDataItem = object
    X, Y: Double;
    // ...
  end;

The fields X and Y are the coordinates of the data point.

Now we write the event handler for OnGetChartDataItem:

procedure TForm1.UserDefinedChartSource1GetChartDataItem(
  ASource: TUserDefinedChartSource; AIndex: Integer; var AItem: TChartDataItem);
begin
  AItem.X := PopulationData[AIndex].Age;
  case Combobox1.ItemIndex of
    0: AItem.Y := PopulationData[AIndex].Total;
    1: AItem.Y := PopulationData[AIndex].Male;
    2: AItem.Y := PopulationData[AIndex].Female;
    3: AItem.Y := PopulationData[AIndex].Ratio;
  end;
end;

AIndex is the index of the datapoint which is queried. Since both chart source and our PopulationData array begin at index 0 we just look into our data array at this index and copy the data to the corresponding elements of the AItem. Note that we pick the y coordinate according to what is selected in the combobox. In this way, the UserDefinedChartSource can provide a multitude of data.

There are still some important things to do:

• Tell the series to use the UserDefinedChartSource instead of the built-in source. For this purpose, we point the series' property Source to UserDefinedChartSource1.
• Tell the UserDefinedChartSource how many data points the external data array contains. We have to enter this number in the property PointsNumber of the UserDefinedChartSource. The data count is known after our procedure LoadPopulationData has finished. So we add a line which sets PointsNumber correctly to the FormCreate method after LoadPopulationData is called.
• Since the UserDefinedChartSource does not know anything about the structure of the external data we have to notify it whenever data are available or have been changed. We do this by calling UserDefinedChartSource1.Reset at an appropriate place. Sometimes you are lucky that some other method may have done this already. But keep in mind that whenever a chart with a UserDefinedChartSource does not behave as you expect there is a chance that you may have forgotten to call Reset. To see what happens let's "forget" to call Reset for the moment.

Fine-tuning

Compile the project in this stage. Not too bad, some flaws, though.

• The labels of the y axis are strange. Why do they jump from 80000000 to 1, and then to 12? This seems to be due to a bug of the Format function for large numbers in some FPC versions. If you have that issue as well, simply change the property LeftAxis.Marks.Format. This string is passed to the Format function to convert numbers to strings. The format specifier "%.0n" for example avoids that conversion error and, additionally, adds nice thousand separators to the labels which makes them much more readable. However, be aware that the "0" cuts off any decimals. Therefore, you should use this format specifier only for large numbers.
• There is a small gap between the y axis and the filled area series. Similarly, the filled area reaches a bit below the x axis. This is caused by the property Margins of the chart. This property is usually quite helpful to create an empty area near the plot axes, free from overlapping symbols and marks labels. But here, it is unfavorable. So just set Chart.Margins.Left and Chart.Margins.Bottom to 0.
• Ah, and the combobox is not working yet. We need to assign a handler for its OnSelect event. What does it have to do? Well, when another combobox item has been selected the UserDefinedChartSource reports a different y value to the series - that's what we want. But how we get the OnGetChartDataItem being called? Just by redrawing the chart! When the series is redrawn it always has to query the data from the chart source, so it will get the updated data automatically. OK - let's call Chart1.Invalidate:

procedure TForm1.ComboBox1Select(Sender: TObject);
begin
  Chart1.Invalidate;
end;

When you compile and select another combobox item you will find that the series does nicely update to the selected category. But the axis is frozen, it should rescale automatically. Is this a bug?

No. A few lines above, it was said that whenever a chart with a UserDefinedChartSource does not behave as expected there is a chance that you may have forgotten to call UserDefinedChartSource.Reset. That's it: we replace the Invalidate by the Reset.

And we can fix another issue: the caption of the y axis should change whenever a different category is selected. For this, we copy the text of the selected combobox item to the chart's LeftAxis.Title.Caption. We add a corresponding line the the combobox OnSelect event handler:

procedure TForm1.ComboBox1Select(Sender: TObject);
begin
  Chart1.LeftAxis.Title.Caption := Combobox1.Items[Combobox1.ItemIndex];
  UserDefinedChartSource1.Reset;
end;

Now switching of data categories by the combobox is working fine.

A minor flaw left: Since our y data do not contain a 0 the y axis does not begin at 0 due to the automatic axis scaling. But it would be nice if there were a zero. For this, we must partially turn off automatic axis scaling. The Chart.Extent defines a rectangle for the minimum and maximum x and y values. When we set Chart.Extent.UseYMin to true, the y axis minimum is taken from the value of Chart.Extent.YMin, and this is 0 by default. Therefore, we have our zero back.

But there is another small problem now: the lowest grid line is drawn on top of the x axis. First of all, the black lines that we see as axes are not the axes themselves, but belong to the frame drawn around the plotting area by default. You can test this by setting Chart1.Frame.Visible temporarily to false. For the axes, there is a property AxisPen where the "true" axis line can be turned on by AxisPen.Visible = true. After after doing this, the x axis is still a dashed line. This is caused by the drawing sequence. By default, the y axis, along with its grid, is drawn after the x axis. If you want to have the x axis drawn first you go to the object tree and drag the x axis above the y axis.

When you compile the chart is perfect.

Source code

For Lazarus v2.1+ you can find the source code of this tutorial project in folder components/tachart/tutorials/population1 of your Lazarus installation. For older versions, copy and paste the following code to the corresponding files.

Project file

program project1;

{$mode objfpc}{$H+}

uses
  {$IFDEF UNIX}{$IFDEF UseCThreads}
  cthreads,
  {$ENDIF}{$ENDIF}
  Interfaces, // this includes the LCL widgetset
  Forms, Unit1, tachartlazaruspkg, population
  { you can add units after this };

{$R *.res}

begin
  RequireDerivedFormResource := True;
  Application.Initialize;
  Application.CreateForm(TForm1, Form1);
  Application.Run;
end.

population.pas

unit population;

{$mode objfpc}{$H+}

interface

uses
  Classes, SysUtils;

type
  TPopulationRecord = record
    Age: Integer;
    Total: Double;
    Male: Double;
    Female: Double;
    Ratio: Double;
  end;

type
  TPopulationArray = array of TPopulationRecord;

procedure LoadPopulationData(const AFileName: String; var Data: TPopulationArray);


implementation

procedure LoadPopulationData(const AFileName: String; var Data: TPopulationArray);

  function StripThousandSep(const s: String): String;
  var
    i: Integer;
  begin
    Result := s;
    for i:=Length(Result) downto 1 do
      if Result[i] = ',' then
        Delete(Result, i, 1);
  end;

var
  List1, List2: TStringList;
  i, j, n: Integer;
  s: String;
  ds: char;
begin
  ds := FormatSettings.DecimalSeparator;
  List1 := TStringList.Create;
  try
    List1.LoadFromFile(AFileName);
    n := List1.Count;
    SetLength(Data, n-2);
    FormatSettings.DecimalSeparator := '.';
    List2 := TStringList.Create;
    try
      List2.Delimiter := #9;
      List2.StrictDelimiter := true;
      j := 0;
      for i:=2 to n-1 do begin
        List2.DelimitedText := List1[i];
        s := List1[i];
        with Data[j] do begin
          if i < n-1 then
            Age := StrToInt(trim(List2[0]))
          else
            Age := 100;
          Total := StrToFloat(StripThousandSep(trim(List2[1])));
          Male := StrToFloat(StripThousandSep(trim(List2[2])));
          Female := StrToFloat(StripThousandSep(trim(List2[3])));
          Ratio := StrToFloat(trim(List2[4]));
        end;
        inc(j);
      end;
    finally
      List2.Free;
    end;
  finally
    FormatSettings.DecimalSeparator := ds;
    List1.Free;
  end;
end;

end.

Unit1.pas

unit Unit1;

{$mode objfpc}{$H+}

interface

uses
  Classes, SysUtils, FileUtil, TAGraph, TASources, TASeries, Forms, Controls,
  Graphics, Dialogs, ExtCtrls, StdCtrls, population, TACustomSource;

type

  { TForm1 }

  TForm1 = class(TForm)
    Chart1: TChart;
    Chart1AreaSeries1: TAreaSeries;
    ComboBox1: TComboBox;
    Panel1: TPanel;
    UserDefinedChartSource1: TUserDefinedChartSource;
    procedure ComboBox1Select(Sender: TObject);
    procedure FormCreate(Sender: TObject);
    procedure UserDefinedChartSource1GetChartDataItem(
      ASource: TUserDefinedChartSource; AIndex: Integer;
      var AItem: TChartDataItem);
  private
    { private declarations }
    PopulationData: TPopulationArray;
  public
    { public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.lfm}

const
  POPULATION_FILE = 'population.txt';

{ TForm1 }

procedure TForm1.FormCreate(Sender: TObject);
begin
  LoadPopulationData(POPULATION_FILE, PopulationData);
  UserDefinedChartSource1.PointsNumber := Length(PopulationData);
  Chart1.LeftAxis.Title.Caption := Combobox1.Items[Combobox1.ItemIndex];
end;

procedure TForm1.ComboBox1Select(Sender: TObject);
begin
  Chart1.LeftAxis.Title.Caption := Combobox1.Items[Combobox1.ItemIndex];
  UserDefinedChartSource1.Reset;
end;

procedure TForm1.UserDefinedChartSource1GetChartDataItem(
  ASource: TUserDefinedChartSource; AIndex: Integer; var AItem: TChartDataItem);
begin
  AItem.X := PopulationData[AIndex].Age;
  case Combobox1.ItemIndex of
    0: AItem.Y := PopulationData[AIndex].Total;
    1: AItem.Y := PopulationData[AIndex].Male;
    2: AItem.Y := PopulationData[AIndex].Female;
    3: AItem.Y := PopulationData[AIndex].Ratio;
  end;
end;

end.

Unit1.lfm

object Form1: TForm1
  Left = 342
  Height = 360
  Top = 128
  Width = 480
  Caption = 'World population'
  ClientHeight = 360
  ClientWidth = 480
  OnCreate = FormCreate
  Position = poScreenCenter
  LCLVersion = '1.1'
  object Panel1: TPanel
    Left = 0
    Height = 36
    Top = 0
    Width = 480
    Align = alTop
    ClientHeight = 36
    ClientWidth = 480
    TabOrder = 0
    object ComboBox1: TComboBox
      Left = 12
      Height = 23
      Top = 5
      Width = 196
      ItemHeight = 15
      ItemIndex = 0
      Items.Strings = (
        'Total population'
        'Male population'
        'Female population'
        'Ratio male/female (%)'
      )
      OnSelect = ComboBox1Select
      Style = csDropDownList
      TabOrder = 0
      Text = 'Total population'
    end
  end
  object Chart1: TChart
    Left = 0
    Height = 324
    Top = 36
    Width = 480
    AxisList = <    
      item
        Grid.Visible = False
        Alignment = calRight
        AxisPen.Visible = True
        Marks.Visible = False
        Minors = <>
      end    
      item
        Grid.Color = clSilver
        Alignment = calBottom
        AxisPen.Visible = True
        Minors = <>
        Title.LabelFont.Style = [fsBold]
        Title.Visible = True
        Title.Caption = 'Age'
      end    
      item
        Grid.Color = clSilver
        AxisPen.Visible = True
        Marks.Format = '%.0n'
        Marks.Style = smsCustom
        Minors = <>
        Title.LabelFont.Orientation = 900
        Title.LabelFont.Style = [fsBold]
        Title.Visible = True
      end>
    BackColor = clWhite
    Extent.UseYMin = True
    Foot.Alignment = taLeftJustify
    Foot.Brush.Color = clBtnFace
    Foot.Font.Color = clBlue
    Foot.Text.Strings = (
      'Source:'
      'http://www.census.gov/population/international/data/worldpop/tool_population.php'
    )
    Foot.Visible = True
    Margins.Left = 0
    Margins.Right = 0
    Margins.Bottom = 0
    Title.Brush.Color = clBtnFace
    Title.Font.Color = clBlue
    Title.Font.Style = [fsBold]
    Title.Text.Strings = (
      'World population'
    )
    Title.Visible = True
    Align = alClient
    ParentColor = False
    object Chart1AreaSeries1: TAreaSeries
      AreaBrush.Color = clSkyBlue
      AreaLinesPen.Style = psClear
      Source = UserDefinedChartSource1
    end
  end
  object UserDefinedChartSource1: TUserDefinedChartSource
    OnGetChartDataItem = UserDefinedChartSource1GetChartDataItem
    left = 552
    top = 152
  end
end