Creating graph of potential temperature

[DaveJ83]

I saw the first attachment in an online module and tried to create it with just 300K potential line. Closest I could get was two lines on the graph. I am looking for help to have just one line of 300K with the two Y axis. Thanks.

Here is my current code

x = dfpt['latitude']
y1 = dfpt['height']
y2 = dfpt['pressure']

fig = plt.figure(figsize=(10,8))
ax = plt.subplot(1, 1, 1)
#want the same exact x range
ax2 = ax.twinx()
ax.plot(x, y1)
ax2.plot(x, y2, color='tab:red')

ax.set_ylabel('Height (m)')
ax.yaxis.label.set_color('tab:blue')
ax.tick_params(axis='y', colors='tab:blue')
#use ax2. ax2 is on top of ax
ax2.spines['left'].set_color('tab:blue')

ax2.set_ylabel('Pressure (mb)')
ax2.yaxis.label.set_color('tab:red')
ax2.tick_params(axis='y', colors='tab:red')
ax2.spines['right'].set_color('tab:red')

ax.set_xlabel('Latitude', fontsize=12)
ax.set_title("Height and Pressure of 300k Potential Temperature\n4/21/25 12z\nSouth to North")

[dopplershift]

You want to use matplotlib's ax.secondary_yaxis() method to create a separate y-axis on the right that has a pair of functions controlling the scaling. Something like:

import matplotlib.pyplot as plt
import metpy.calc as mpcalc
from metpy.units import units

x = np.linspace(90, 0, 100)
hgt = 3 * (x/90)**2

fig = plt.figure(figsize=(10,8))
ax = plt.subplot(1, 1, 1)
#want the same exact x range
ax.plot(x, hgt)

ax.set_ylabel('Height (m)')
ax.yaxis.label.set_color('tab:blue')
ax.tick_params(axis='y', colors='tab:blue')

ax2 = ax.secondary_yaxis('right', functions=(lambda h: mpcalc.height_to_pressure_std(h * units.km).m,
                                             lambda p: mpcalc.pressure_to_height_std(p * units.hPa).m))
ax2.set_ylabel('Pressure (mb)')
ax2.yaxis.label.set_color('tab:red')
ax2.tick_params(axis='y', colors='tab:red')
ax2.spines['right'].set_color('tab:red')

ax.set_xlabel('Latitude', fontsize=12)
ax.set_title("Height and Pressure of 300k Potential Temperature\n4/21/25 12z\nSouth to North")

There's a little more complexity because matplotlib seems to be choking on/dropping the units.

[DaveJ83]

Thanks. I copied that and changed x and hgt to my data. I get ValueError: Axis limits cannot be NaN or Inf
Here are the values for x and hgt

[30.34 32.32 34.84 37.23 41.13 44.85 46.76]
[1526 1436 1510 2418 2134 3197 3309]

#x = np.linspace(90, 0, 100)
#hgt = 3 * (x/90)**2
x = dfpt['latitude'].to_numpy()
hgt = dfpt['height'].to_numpy()
print(x)
print(hgt)

fig = plt.figure(figsize=(10,8))
ax = plt.subplot(1, 1, 1)
#want the same exact x range
ax.plot(x, hgt)

ax.set_ylabel('Height (m)')
ax.yaxis.label.set_color('tab:blue')
ax.tick_params(axis='y', colors='tab:blue')

ax2 = ax.secondary_yaxis('right', functions=(lambda h: mpcalc.height_to_pressure_std(h * units.km).m,
                                             lambda p: mpcalc.pressure_to_height_std(p * units.hPa).m))
ax2.set_ylabel('Pressure (mb)')
ax2.yaxis.label.set_color('tab:red')
ax2.tick_params(axis='y', colors='tab:red')
ax2.spines['right'].set_color('tab:red')

ax.set_xlabel('Latitude', fontsize=12)
ax.set_title("Height and Pressure of 300k Potential Temperature\n4/21/25 12z\nSouth to North")