class Fl_Hor_Fill_Slider : public Fl_Slider

This subclass of Fl_Slider sets type() to FL_HOR_FILL_SLIDER. The bar is drawn between the current position and whichever end has the smaller minimum/maximum() value. So you can make it fill the left or right by inverting the minimum and maximum.

Fl_Hor_Fill_Slider(int x,int y,int w,int h,const char * = 0);