Add alt_text option to save_plot_as_png

[ruaridhg]

Checked with exiftool -PNG:Alt <image_name.png>

[dstansby]

Nice! If alt-text is added to the .png metadata file this way, and the .png file is added to a web page (ie, https://heftieproject.github.io/zarr-benchmarks/), does web browser "know" this is where to find the alt-text, and display it when moused over (or say it when a screen reader is being used)? I assumed that we might have to manually add the alt-text to the markdown file, but if we can semi-automate it like this that would be a bit easier.

[dstansby]

Following on from our meeting, out of interest where did you get inspiration for this implementation from? I'm intrigued if adding alt-text to .png files in this way is some sort of standard.

[ruaridhg]

Following on from our meeting, out of interest where did you get inspiration for this implementation from? I'm intrigued if adding alt-text to .png files in this way is some sort of standard.

https://stackoverflow.com/questions/58399070/how-do-i-save-custom-information-to-a-png-image-file-in-python
It may just be if you're using Pillow so probably not standard. I added alt-text to the main PR #79 instead of using this method.

[dstansby]

Should we close this in favour of your changes in the other PR then?