Widget not shown in QMainWindow - c++

Question

Good evening,
the aim is to have a mainwindow (created without Designer but by coding) with three sections next to each other:
a list of data points (vector)
statistics about the data points
a graphical summary (histogram)of the data
I have started to create my own widget do draw a diagram (just a line for test purposes so far).
However, while the tableview widget is shown, the "diagram" widget (instance of class histogram) is not shown (should be a third vertical column).
A very similar problem in Stackoverflow gave me some direction. But while it helped me to display the widget with the table, I did not figure out how to show my custom widget.
post: widgets not shown in qt main window
I have also checked literature (Summerfield and Qt4 Hui Entwicklung mit C++ by Jürgen Wolff), but they only have examples with only one central widget.
#include "mainwindow.h"
#include <QApplication>
#include <QDebug>
MainWindow::MainWindow(QMainWindow *parent, Qt::WindowFlags flags) : QMainWindow(parent, flags)
{
mainWidget = new QWidget(this);
setCentralWidget(mainWidget);
tableWidget = new QTableWidget(mainWidget); // QTableWidget to display the data vector
//...
// here comes code to fill the table...
//...
// result labels
lbl_sampleSize = new QLabel("sample size");
lbl_meanValue = new QLabel("mean");
lbl_sigma = new QLabel("sigma");
lbl_andersonDarling = new QLabel("Anderson Darling");
lbl_pValue = new QLabel("p-value for Anderson-Darling");
rightLayout = new QVBoxLayout(); // a vertical layout to contain labels
rightLayout->addWidget(lbl_sampleSize);
rightLayout->addWidget(lbl_meanValue);
rightLayout->addWidget(lbl_sigma);
rightLayout->addWidget(lbl_andersonDarling);
rightLayout->addWidget(lbl_pValue);
rightLayout->addStretch();
//diagram
diagram = new Histogram(mainWidget);
mainLayout = new QHBoxLayout(mainWidget);
mainLayout->addWidget(tableWidget,0);
mainLayout->addLayout(rightLayout,0);
mainLayout->addWidget(diagram, 0);
//mainWidget->setLayout(mainLayout);
}
screenshot:
remark:
with this code
//diagram
diagram = new Histogram();
mainLayout = new QHBoxLayout(mainWidget);
mainLayout->addWidget(tableWidget,0);
mainLayout->addLayout(rightLayout,0);
//mainLayout->addWidget(diagram, 0);
//mainWidget->setLayout(mainLayout);
diagram->show();
I was able to create a separate Widget with the test diagram.
(removed the parent information and called diagram->show())

Answer 1

Just set the minimum size (QWidget::setMinimumSize()):
diagram->setMinimumSize(100, 100);
More control over the widget sizing can be achieved through its size policy.