First Graph

For creating a graph, we use the class TGraph by writing the following line of code into a new script file called graph.C:

 TGraph *gr = new TGraph();

There are several ways to fill a graph. We can for example hand over data in form of float or double arrays. Let us define two arrays x and y in the following way:

double x[5] = {0,1,2,3,4};
double y[5] = {0,1,4,9,16};

In this case, we have to change the line according to

 TGraph *gr = new TGraph(5, x, y);

Now we can draw the filled graph similar to how we have drawn the previous histogram. The code looks as follows:

void graph()
{
    double x[5] = {0,1,2,3,4};
    double y[5] = {0,1,4,9,16};

    TGraph *gr = new TGraph(5, x, y);

    TCanvas *c1 = new TCanvas();
    gr->Draw();
}

As we can see, the graph contains a line that connects all inserted data points. The result is one side of a parabola as expected. The standard title is simply "Graph". In contrast to a histogram, no statistic box is displayed, since it would not make a lot of sense for graphs. We can also fill values into a TGraph by using the member function SetPoint(). The first argument of this function has to be an integer that denotes the incremented number of the point to add. The second and third arguments are x and y coordinate. Just as an example, we can rearrange our previous example as follows:

void graph()
{
    TGraph *gr = new TGraph();

    gr->SetPoint(0, 0, 0);
    gr->SetPoint(1, 1, 1);
    gr->SetPoint(2, 2, 4);
    gr->SetPoint(3, 3, 9);
    gr->SetPoint(4, 4, 16);

    TCanvas *c1 = new TCanvas();
    gr->Draw();
}

After plotting this graph, we will find the exact same result as shown before. If we do not specify any drawing options for a graph, the graph contains straight lines that connect all data points. If we want to change that, we have to insert our desired drawing options as a string directly into the Draw() function of TGraph. The standard configuration is

 gr->Draw("AL")

The letter A stands for axis and always has to be inserted when drawing a new graph. The letter L is used for lines. If we want to draw a smooth curve through the given data points, we have to replace L with C for "curved". In addition, we can draw the points by inserting e.g. an asterisk * into the draw option string:

 gr->Draw("AC*")

Another important draw option is "P" which stands for "points". If you write for example

 gr->Draw("AP");

and run your program, the points will not be visible. In order to fix this issue, you first have to define the marker style for your graph. This you can do with the member function of TGraph called SetMarkerStyle(). The size of the marker you can additionally change using the function SetMarkerSize(). The marker style has to be defined using an integer value, whereas the size can be fixed with a float value. Going back to our example before we can now use the following code

void graph()
{
    TGraph *gr = new TGraph();

    gr->SetPoint(0, 0, 0);
    gr->SetPoint(1, 1, 1);
    gr->SetPoint(2, 2, 4);
    gr->SetPoint(3, 3, 9);
    gr->SetPoint(4, 4, 16);
    
    gr->SetMarkerSize(1.5);
    gr->SetMarkerStyle(20);


    TCanvas *c1 = new TCanvas();
    gr->Draw("AP");
    c1->Print("root_graph_points.png");
}

to display a graph with bullets without any connections between the data points.