The documentation states:
Pressing QwtEventPattern::MouseSelect1 or QwtEventPattern::KeySelect1 starts the selection and selects the first point, or appends a point. Pressing QwtEventPattern::MouseSelect2 or QwtEventPattern::KeySelect2 appends the last point and terminates the selection
However, that intuitive mapping is not implemented.
Please find attached a patch against 6.01 to achive that.