Height Scale for SkewT

[blue-jaye-121]

Description Of Changes

Added new argument to metpy.plot.SkewT function for an optional secondary y-axis of height calculated from metpy.calc.pressure_to_height_std(). Defaults to 'False'; axis can be accessed as a matplotlib axis using 'heightax'.

Checklist

• [x] Closes #262
• [x] Tests added
• [x] Fully documented

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:white_check_mark: blue-jaye-121
:x: Jaye Norman

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[blue-jaye-121]

I reverted the skewT file to the best functioning version that I had. This works beautifully - unless you add a ylim that is greater than 1013.25 hPa. Then because the secondary axis uses the same log scale as a primary axis, it doesn't work and won't reset the limits. Here's a couple examples:

1. Works great with set_ylim(1000, 100)
2. Works great with set_ylim(1000, 400)
3. Does not work with set_ylim(2000, 100) - it keeps the limits the same as the default