Why the NameError?

[Mark17]

The problem with the code above is .twinx() needs to be an attribute of ax and plot is a method (I think I have the terminology correct) of that.

This works:

fig, axs = plt.subplots(1,2)
axs[0].plot(sample_df['Spread_Price'],color='red')
axs[0].twinx().plot(sample_df['SPX']) 
axs[1].twinx().plot(sample_df['Spread_Price']) 
axs[1].plot(sample_df['SPX'],color='red')
plt.tight_layout()
plt.show()

If I want to switch around primary and secondary axis, then I just need to switch which Series follows .twinx().

If I figure out what's causing the NameError then I'll report back. It's not an issue now, though, since I have figured out this syntax (I can therefore run the whole program).