The configuration option for Waterfall Chart Type is not currently available with the Multipurpose Chart control. If you feel that this should be an included functionality out of the box, please submit an Enhancement request here as IBM now owns the SPARK UI Toolkit. (Currently included with BPM 8.5.7 CF201706 and higher)
If you have an urgent requirement for this functionality, please feel free to contact our Sales Team regarding an Engagement in developing a custom control or enhancing an existing one to fit your needs.
Also, please be advised that we do offer Expert Services on a monthly subscription basis for guidance in implementation and development.