Hi Gareth Whiteley,
You should be able to use the Formatting option, at the left of your sheet.
Could you please share your workbook (.twbx) so I can help you with that?
Lénaïc RIÉDINGER, Global Community Engineer Tableau
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There currently is no way to (easily) format lines in a chart. Though, I would encourage you to head over to the Ideas forum and upvote this idea.
You also might want to read through this blog post by Jonathan Drummey wherein he explores three ways to get Tableau to do what you want.
If that doesn't do it, I have a few possible solutions that might work for you, though none of them are different line styles:
1. Create a calculated field that is "school + ***". Add that to Color property on the Marks card. Manually update the colors so that each school has the same color with a particular *** being a darker or lighter shade.
2. Add *** to the Detail property on the Marks card. This will create twice as many lines, but not more colors. The downside of this is you wouldn't be able to visually differentiate between sexes at a glance unless you add a label. Of course by hovering you could easily see which is which.
3. Add *** to the Size property on the Marks card. The result of this will be twice as many lines, but easily differentiated because one would be thick and the other thin. The major downside of this, of course, is that one of the sexes now appears less prominently than the other and could be easily overlooked.
I hope that helps!
Different line styles is not an option in Tableau
(and I mean the lines connecting dots on a line chart).
Typically one would opt to use color and / or size
(line thickness) to differentiate on a view.
Highlighting could help, too.
So you may want to use Gender (blue pill)
on Color together with your Schools (blue pill).
Just place both pills on Details and use Color for both.
Alternatively, you may want to make a small multiples
using your School and Gender dimensions as a grid.