Qt Creator can't locate Qwt plugin

I want to use the Qwt plugin in Qt Creator, but I have troubles installing it. I work with Debian 10, Qwt 6.1.6, Qt Creator 4.8.2 and Qt 5.11.3. I followed these instructions: https://qwt.sourceforge.io/qwtinstal...wtinstall-unix

These are the lines I added to .bashrc:

QT_PLUGIN_PATH="/usr/local/qwt-6.1.6/plugins:$QT_PLUGIN_PATH" export QT_PLUGIN_PATH

I added this to my .pro file (this is a new project I started):

INCLUDEPATH += /usr/local/qwt-6.1.6/include

LIBS += -L/usr/local/qwt-6.1.6/lib -lqwt

CONFIG += qwt

However, I still don't see the new widgets in the UI form (after restarting Qt creator). I think Qt Creator hasn't located the Qwt plugin yet.

I see some explanation about QWT_INSTALL_PLUGINS in the config.pri of the downloaded Qwt, but I am not sure how I should edit the .bashrc file.