update: chart thumbnails (better resolution and ~45% smaller size)

[vostrnad]

Restores chart thumbnails to their original 1296 x 650 resolution and reduces their total size by ~45%. The original resolution works better for the 16:9 ratio they're displayed in as this crops them horizontally (trimming off time ranges at the edges) instead of vertically (trimming off graph values).

[0xB10C]

Definitely in favor of merging this, but would be good for me to figure out why my images are larger... Is the optipng you're using more aggressive by default?

[0xB10C]

optipng -h prints -o <level> optimization level (0-7) [default: 2] for me.

[vostrnad]

I don't know if optipng has anything to do with it, as mentioned in #118 (comment) running it on the existing thumbnails barely made a difference. One thing I did notice is that I'm using optipng version 7.9.1 while the version used by the GitHub workflow is 0.7.8 which is apparently from 2017 2023. It could be that the running the old version and then the new one produces worse results than running the new version outright. Or it could be that the difference occurs at the point of generating the images due to OS or hardware differences, in which case I don't hope to ever understand it.

I tested different optimization levels, for most thumbnails the higher levels produce a byte-for-byte identical result, and the rest are improved by less than 1%.

[0xB10C]

I plan to experiment a bit if I can reproduce this on Windows.