Use JUCE as much as you like, but please, stay away from those 'out-of-the-box' horizontal sliders it seems to prioritize (given how they swarm in the many VSTs made with JUCE).

If sliders are vertical in the real world, there’s a reason for it that Bruce “Tog” Tognazzini summarizes perfectly:

“Most of us […] have our forarms mounted on a pivot we like to call our elbow. That means that moving our hand describes an arc, rather than a straight line. Demanding that pivoted people move a mouse pointer along in a straight line horizontally is just wrong.”

Source: https://www.asktog.com/columns/022DesignedToGiveFitts.html