Leader line elbows

[sam-ctrl]

This is a bit of a can of worms, but after the slack conversation about having "elbowed" (orthogonal? horizontal-vertical-horizontal) leader lines, this is what I though we should do.

It now only draws leader lines if they are needed, and it adds some margin to account for them.

It has some special handling for "early ending" series. You can set it with the "labelLocation" to either draw it off to the right vertically aligned with the other labels (labelLocation: "margin") with a leader line (labelLocation: "marginLeader"), or next to the last point (labelLocation: "lastPoint")

You can also set the style of the leader lines to be dashed or solid, and to have the colour of the corresponding series or a standard (configurable) colour.

The leader lines now also point to the vertical centre of the direct labels

It might be a bit too configurable (have added a fair bit to the config files) and could maybe be trimmed down a bit.

When there are many leader lines (>4 in a cluster), it's can a bit difficult to see what is what, this could be helped maybe by offsetting the vertical parts of the leader lines (a bit how ourworldindata do it) but I think it would add significant complexity to something that is already quite complex!

I might implement the "symbols" aspect that was also discussed as a different PR shortly

[henryjameslau]

I bet this is gonna break slope charts #387

[sam-ctrl]

I bet this is gonna break slope charts #387

Could we have a createDirectLabelsSlope() function for now, and then work on integrating the two once the separate PRs are merged?

[henryjameslau]

I feel like this is more confusing when they overlap so we might need to have to add staggering

[broadpeter]

In this example would be better if the category 2 went down. With less padding and downward shifts it would be a lot better. Possible only needing a leader for the top most 1 or 2.

[henryjameslau]

Let's use the ELS algorithm

[nrennie]

Is this PR still "Ready to merge" if we're going to use the ELS algorithm?