Add colour as third visualisation factor for line and bar charts

[gannebamm]

We intend to fund this development as part of an upcoming tender process. This feature request shall start a discussion about how the developed functionality could get upstreamed to the main project.

Issue: The currently enabled line and bar chart widgets are limited by using only x and y parameters. For example, you are not able to chart time x value x type in a line chart with the factor type used as a colour. This additional parameter would result in a multi-line chart. See this current example:

Current line chart implementation for charting time and AVG value for a dataset. You can only pick one colour for the graph resulting in a single line chart.

Objective: To enable grouping of datasets in charts the colour for each line/bar should be set as attribute-based on categorial field values. See this example of the same dataset rendered as Apache Superset line chart:

Example of a line chart with colour as additional grouping level which results in multiple lines.

The metric ‘art’ is used to group by and a colour ramp is fetched to create a multi-line chart.
The same functionality shall be provided for bar charts which will result in stacked charts, like drafted here:

Example of a stacked barchart

[AlexGacon]

Similar to #806 , I don't know if it is not going to far into dataviz feature.

[gannebamm]

There are some exciting features shown in this presentation: https://speakerdeck.com/simboss/developing-with-mapstore-creating-a-custom-dashboard-to-map-crime-data

Maybe some code snippets can be re-used for this feature. See Slide 36

[JohannesSchnell]

@giohappy We saw that this feature is already available for stacked bar plots (choose custom colors, with click on the pen-symbol next to it then a new window opens where you can pick a classification attribute and then manually pick values and colors to be displayed. Under "extended Options" you can switch between a stacked or grouped bar plot).
Are there any plans on your side to add this classification process to line plots (e.g. in the form of a multiline plot) as well?
If not so we would implement this by adapting the above described workflow