Checkerboard relative scale is percentage rather than absolute-value based

[sgress454]

Fleet versions 4.85.0

Web browser and operating system:

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💥  Actual behavior

Charts that are colored using a relative scale are doing so based on the percentage of hosts in a bucket, rather than the number of hosts. So in certain cases where the numbers may vary a good amount but they still represent a small percentage of the whole, the chart may only use one or two colors.

🛠️ Expected behavior

A relative chart should always show all colors as long as there are at least 5 distinct values in the chart.

🧑‍💻  Steps to reproduce

• Start an instance with 10 hosts, and use the chart-backfill tool to backfill uptime data for the last 30 days.
• Add an additional 990 hosts to the instance without backfilling
• Note that the "hosts online" chart is all one color (since every square will represent 1% (rounded up) of the 1000 hosts)

[sgress454]

@lukeheath @noahtalerman @AndreyKizimenko I added this to 4.85.1 because the prior fix (#44959) didn't make it into 4.85.0. That one was somewhat more important (without it, the chart starts to show empty boxes in the left column incorrectly after 30 days) but both that one and this had fixes to make the chart colors spread more evenly, especially when a low # of hosts are online out of the total # available. Up to y'all whether it warrants patch, cherry-pick to 4.86, or just release in 4.87.