Charts with projections do not do multiple entities well

[larsyencken]

Background

Sometimes our data sources include projections into the future. Grapher handles this by using two indicators, one for the "stem" of the chart containing actual measurements, and one for the projections. The projection indicator has isProjection: true set, causing it to be displayed as a dotted line.

Problem

Our approach makes all lines with projections into multi-indicator charts. But when charts are both multi-indicator and multi-entity, then every line needs a double-label, which is super ugly.

Impact

This tends to impact only a few charts, but quite important ones.

We have workarounds, but they require us to either

• (a) configure the chart to disallow multiple entity selection (e.g. population-growth-rates); or
• (b) fold projections into a single indicator and hide the fact that it's a projection (e.g. Population and Demography explorer)

Possible approaches

• We could disable dual-labelling of lines, when a chart has only two indicators and one is a projection
• We could provide the ability to mark-up that an indicator is a projection after a certain year (e.g. "projectionAfter": 2023)

[larsyencken]

/cc @lucasrodes

[larsyencken]

Here's a good example of a benefit of keeping projections in different indicators, at least as one of our options: https://ourworldindata.org/grapher/comparison-of-world-population-projections

[marcelgerber]

Another related issue we have is that currently, there is a visible gap between when the observed data ends and when the projections begin.

[danyx23]

The problem of upper and lower confidence intervals is also a bit related (two more time series that are closely related but lead to some odd behaviour e.g. in facetting).

Probably it makes sense to keep these as different variables as they can then also have different sources (e.g. the historic data might come from a different institute than the projection?).

The relationship of different variables/indicators to each other is something that Grapher is very bad at. If it were better with this then this could also have big benefits for doing drillup/-down.