Author Topic: Scalling  (Read 895 times)

Offline Oskarschlyter

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« on: June 20, 2016, 10:31 AM »
Havet a problem with a flowchart. Two streams a and b both are measured in tons. Since a is much bigger then b the automatic scalling hides b. Is it possible to scale a against a and b against b ?

Offline pbeilschmidt

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Re: Scalling
« Reply #1 on: June 21, 2016, 10:01 AM »
Hello Oskar,

this is a general problem in Sankey diagrams. When you have two flows that are of very different dimensions (e.g. 100.000 tons vs. 2 tons), it is difficult to see the small flow, since it is rather thin compared to the large dominant flow. One could use a different scale to make the smaller flow wider, but then the large flow might spoil the whole picture.

So what to do? You could decide to not show both flows in one diagram, or decouple the two flows so that they don't show on the same scale any more (I think this is what you mean by "scale a against a and b against b"). In this case, however, you are basically breaking the most important "law of Sankey diagrams" that flows are to scale. You should at least put a note in the diagram to inform the reader/viewer. To have two different scales for a and b just define another unit type (via Controller > Edit Unit Types > New Unit Type). You can define the same unit "ton" for it, or another one (e.g. 'Mt').

I would, however, suggest that you have a look into two other options before you define a second unit type for mass that scales differently:

1 - 'Lower flow threshold': You can turn on this option in the document properties. If you set the value to e.g. 2 pixels (px), then the small flow will be show with a mimimum of two pixels. It will have a minimum width, even though in reality it would have to be shown thinner.

2 - 'Cut Off': If you switch the Scale Mode in the Controller to 'Absolute Quantity of Each Unit Type' you can enter a cut off value for the unit type in question in the field below the scale factor. Set the flag for 'Cut Off'. As a result the flow will not scale wider than the absolute flow. Example: If you define 10.000 tons as the cut off value (10.000 shown at, let's say, 25 px), then any flow larger than 10.000 tons will not be shown wider than 25 px. Instead a hatching pattern will appear on the arrow to signal that this flow is not to scale any more.

So for extremely small and extremely large arrows, you have these two options described above: Either at the lower end of the scale, or at the upper end. Always put a note in the diagram to warn your readers that some flows in the diagram are not to scale any more.

Peter Müller-Beilschmidt
e!Sankey Forum Moderator
ifu Hamburg GmbH