Add angular units

[enourbakhsh]

It would be very useful for astronomers to have a scale bar showing angular scales such as degree (°), arcmin (') and arcsec ('').

For example, this can be done by adding the so-called pixel scale in the units of arc seconds per pixel (or even more available units) as an input. For example pixel scale = 0.2 implies that each pixel spans 0.2 arcsec. Here is the best I could do with the current version of matplotlib-scalebar.

[ppinard]

Thank you for this request. I made a first implementation in the branch angular. There is a new dimension Angular where the units can either be degree (°), minute of arc (′, 1/60 of °) or second of arc (′′, 1/3600 of °).

Here is how it looks like:

Please let me know if this addresses your issue. If so, I will merge the branch and make a new release.

[ericpre]

What's about using the pint library to manage the units and prefixes?

[ppinard]

@ericpre Regarding using pint, could you explain a bit more what you had in mind? I am not sure I would like to add a dependency to the project for something as simple as defining the conversion factor between units. While pint could be used to discover possible units within a unit group (e.g. [length]), I think the list of units available for a scale bar needs to be customized, hence the Dimension class. For instance, having "mile", "nautical mile" and "us nautical mile" as available units would be confusing for a scale bar.

[ericpre]

This was a quick suggestion and I haven't thought properly about it but I find it convenient to use! I don't have anything particular in mind other than it is very convenient to get the appropriate prefix and a nice formatting, for example:

from pint import UnitRegistry
ureg = UnitRegistry()
length = 1550 * ureg.nm
print(f'{length.to_compact():~}')

Pint does more than what is needed here but it is pure python library and has no dependency, so it does not cost much to add it as dependencies!

[ppinard]

Code merged. Please re-open if it doesn't address the original issue.