docs: Fix missing code block fence for hidden_legend_constraints

[ClementTsang]

Two things:

1. Minor thing that I noticed when using this function. Seems to look fine in documentation, but looked weird on stuff like VS Code.

2. When using GraphType::Line, if the dataset is empty, you can end up with a subtraction underflow when you do the length - 1.

[ClementTsang]

EDIT: This was originally just a documentation fix but I found the GraphType::Line bug soon after... I guess kill two birds with one PR.

[fdehau]

I think that your fix is the object of an other dedicated PR (#283). So I would happily take the documentation fix in this PR and resolve the overflow bug in the other one.

[ClementTsang]

Ah, my bad. Forgot that I pushed that change temporarily to use in my Cargo. I'll revert the third commit.

The second commit is different from that PR; this is more of a fix for if dataset.data is empty, as for i in 0..dataset.data.len() - 1 {...} occasionally seems to fail otherwise due to underflow.

I can revert that commit as well and do it in a separate PR if that's better.

[fdehau]

Thanks. No you can left the other bug fix here that's fine. I've left a comment and it would be nice if you could add a small test covering this case as well.

[fdehau]

Thanks !