Add year as time unit to Gantt chart

[Venthe]

Currently there is no possibility to use years in a long running project.

I'd suggest adding this both as a scale option and as a noun.

@startgantt
projectscale yearly
[java 8 support] lasts 3 years
@endgantt

Additionaly, this could be thought as general "add more time nouns" request, following:

• https://github.com/plantuml/plantuml/issues/375

In my opinion, if possible, Gantt should support date types for at least months, quarters and years

[Venthe]

@arnaudroques Would you be open for a PR?

[arnaudroques]

Yes, we are open for PR. Note that the main issue is that months, quarters and years do not have a fixed duration in days.

[Venthe]

I suggest two approaches (Please note that I'm still digging through code):

• RELATIVE: If start date for the project is set, calculate relative date with standard tools
• ABSOLUTE: If start date for the project is not set, then use average calendar data (Average month is 30.436875 days)

See:

• https://www.britannica.com/science/time/Lengths-of-years-and-months
• https://www.quora.com/What-is-the-average-number-of-days-in-a-month