adjust the discharge figures for the longitudinal comparison

[kdberends]

Is your feature request related to a problem? Please describe.
At this moment the data in the figures doesn't cover to total discharge range

Describe the solution you'd like
There is a strong desire from at least RWS-ON to plot the waterslevels on the different branches linked to the discharge at Lobith. At this moment the data in the figures doesn't cover to total discharge range. For example at the IJssel branch the maximum projected discharge is 1600 m3/s which is equivalent to ~13000 m3/s at Lobith.  

Would it be an option to translate the longitudinal figure displaying dates to the discharge at Lobith? That way to total range can be covered. 

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
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