Unable to setup Qcustomplot

Hello,

I am a newbie for Qt. Could some one help to overcome the errors.

D:\Programs\Qt\build-test_plot-Desktop_Qt_5_5_1_MinGW_32bit-Debug\ui_mainwindow.h:30: error: 'QcustomPlot' does not name a type
QcustomPlot *widget;

D:\Programs\Qt\build-test_plot-Desktop_Qt_5_5_1_MinGW_32bit-Debug\ui_mainwindow.h:42: error: 'widget' was not declared in this scope
widget = new QcustomPlot(centralWidget);

D:\Programs\Qt\build-test_plot-Desktop_Qt_5_5_1_MinGW_32bit-Debug\ui_mainwindow.h:42: error: expected type-specifier before 'QcustomPlot'
widget = new QcustomPlot(centralWidget);

When I am trying to follow the steps to setup Qcustomplot as mentioned in the http://www.qcustomplot.com/index.php...ials/settingup
I am getting the above errors.
Could someone help to overcome these errors.