Coherence time axis

[CommonClimate]

Problem : the original code was creating tau based on the left series, not the right. This is now fixed, using the "common_time" rule. Additionally, the dashboard restricts the view to the period of overlap. Here's a MWE:

import pyleoclim as pyleo
SOI = pyleo.utils.load_dataset('SOI')
NINO3 = pyleo.utils.load_dataset('NINO3')
coh_wwz = NINO3.wavelet_coherence(SOI, method='wwz')
fig, ax = coh_wwz.dashboard()

The result should look like this:

this should address #479 . TODO: update tutorials