Ensure x-axis in stacked bar charts is linear

[sophiamersmann]

fixes #1865

Problem

If observations are unevenly spaced, stacked bar charts end up with a non-linear x-axis.

Solution

To enforce uniform spacing along the axis, this PR implements Marcel's suggestion that involves computed the greatest common divisor. In Marcel's words, the gist of it is:

given sortedYears = [1940, 1960, 1970, 1985]
compute deltas = [20, 10, 15]
compute gcd(deltas) = 5
=> the "minimum display interval" is 5 in this case
=> show 1940, 1945, 1950, 1955, 1960, 1965, ..., 1985 on the x axis

Weirdnesses

The SVG tester comes back with one problematic chart:

Problem: This chart shows monthly data but since we currently have no native way to represent monthly data, this data is treated as daily data where only the 15th of every month is assigned a data point. Working with daily measurements becomes a problem here because slightly different deltas (number of days between measurements, i.e. 28, 30 or 31 days) lead to a small greatest common divisor (=1), and this leads in turn to many fake data point added (one for every day!).

Possible Solutions: This would be solved by making monthly data a first-class citizen as suggested in this issue. Going forward, we could either:

• Bench this PR and wait for Support for more time intervals than yearly and daily #2007 to be implemented
• Accept that this chart is going to look weird until we implement Support for more time intervals than yearly and daily #2007
• Wait for Support for more time intervals than yearly and daily #2007 to be implemented but add some ugly/hacky code to make this chart look ok in the mean time
• Add an option to opt out of the new behaviour of enforcing linear x-axis

[marcelgerber]

Cool, this is working well already!

• Accept that this chart is going to look weird until we implement Support for more time intervals than yearly and daily #2007
• Wait for Support for more time intervals than yearly and daily #2007 to be implemented but add some ugly/hacky code to make this chart look ok in the mean time

I think we should do one of these two. The ugly hack could just be that we disable this behaviour for charts with daily data.

[marcelgerber]

That's quite a lot of comments! Sorry, totally didn't want to roast your code; there's just a bunch of edge cases in here that don't work well, and a bunch of perf improvements for the common case.

[sophiamersmann]

Thanks for the review!

The ugly hack could just be that we disable this behaviour for charts with daily data.

Makes sense! Let's do that for the moment then :)