But you referred to the designer. Can you please tell me how this can be done there as well?

It's already done in the tutorial. See the "Promote to..." step.

I followed the steps in the integration instructions as posted above but still, VS does not introduce the necessary code.
Maybe I missed a step - those are instructions for the Qt Creator, not designer:
https://www.qcustomplot.com/index.php/tutorials/settingup
But you referred to the designer. Can you please tell me how this can be done there as well? Thanks.

But the problem is that the member "plot" is not generated by VS2017

So, you are using QtDesigner to design your UI - is that correct?
This "plot" variable will not be generated magically - you have to put it in QtDesigner like any other widget in order to be able to access it via ui->.

https://www.qcustomplot.com/index.php/tutorials/settingup

To no avail - still, building my VS2017 project gives

Severity	Code	Description	Project	File	Line	Suppression State
Error	C2039	'plot': is not a member of 'Ui::SampleRenderer'	

void setupUi(QMainWindow *MainWindow)
    {
        .....
        plot = new QCustomPlot(centralWidget);

But I don't have this code generated for me....

Thanks for replying.

The variable "ui" is an object provided by Qt, such as

private:
    Ui::SampleRenderer ui;

which is defined here (generated code):

(ui_SampleRenderer.h)
namespace Ui {
    class SampleRenderer: public Ui_SampleRenderer {};
} // namespace Ui

which is turn is then included in my class:

#include "ui_SampleRenderer.h"

and finally

ui->plot->xAxis->setRange(0, 6, Qt::AlignCenter);

But the problem is that the member "plot" is not generated by VS2017, which is not surprising due to the fact that there is nothing in the ".ui" file to mandate it.
Can I use QCustomPlot separately?
Can you offer an alternative approach?

How exactly are you trying to use QCustomPlot in your application ?